diff --git a/include/fmt/chrono.h b/include/fmt/chrono.h
index 9837c668b123d..96096caf72318 100644
--- a/include/fmt/chrono.h
+++ b/include/fmt/chrono.h
@@ -799,974 +799,970 @@ template <typename Derived> struct null_chrono_spec_handler {
   FMT_CONSTEXPR void on_tz_name() { unsupported(); }
 };
 
-struct chrono_format_checker : null_chrono_spec_handler<chrono_format_checker> {
-  FMT_NORETURN void unsupported() { FMT_THROW(format_error("no date")); }
+struct tm_format_checker : null_chrono_spec_handler<tm_format_checker> {
+  FMT_NORETURN void unsupported() { FMT_THROW(format_error("no format")); }
 
   template <typename Char>
   FMT_CONSTEXPR void on_text(const Char*, const Char*) {}
+  FMT_CONSTEXPR void on_year(numeric_system) {}
+  FMT_CONSTEXPR void on_short_year(numeric_system) {}
+  FMT_CONSTEXPR void on_offset_year() {}
+  FMT_CONSTEXPR void on_century(numeric_system) {}
+  FMT_CONSTEXPR void on_iso_week_based_year() {}
+  FMT_CONSTEXPR void on_iso_week_based_short_year() {}
+  FMT_CONSTEXPR void on_abbr_weekday() {}
+  FMT_CONSTEXPR void on_full_weekday() {}
+  FMT_CONSTEXPR void on_dec0_weekday(numeric_system) {}
+  FMT_CONSTEXPR void on_dec1_weekday(numeric_system) {}
+  FMT_CONSTEXPR void on_abbr_month() {}
+  FMT_CONSTEXPR void on_full_month() {}
+  FMT_CONSTEXPR void on_dec_month(numeric_system) {}
+  FMT_CONSTEXPR void on_dec0_week_of_year(numeric_system) {}
+  FMT_CONSTEXPR void on_dec1_week_of_year(numeric_system) {}
+  FMT_CONSTEXPR void on_iso_week_of_year(numeric_system) {}
+  FMT_CONSTEXPR void on_day_of_year() {}
+  FMT_CONSTEXPR void on_day_of_month(numeric_system) {}
+  FMT_CONSTEXPR void on_day_of_month_space(numeric_system) {}
   FMT_CONSTEXPR void on_24_hour(numeric_system) {}
   FMT_CONSTEXPR void on_12_hour(numeric_system) {}
   FMT_CONSTEXPR void on_minute(numeric_system) {}
   FMT_CONSTEXPR void on_second(numeric_system) {}
+  FMT_CONSTEXPR void on_datetime(numeric_system) {}
+  FMT_CONSTEXPR void on_loc_date(numeric_system) {}
+  FMT_CONSTEXPR void on_loc_time(numeric_system) {}
+  FMT_CONSTEXPR void on_us_date() {}
+  FMT_CONSTEXPR void on_iso_date() {}
   FMT_CONSTEXPR void on_12_hour_time() {}
   FMT_CONSTEXPR void on_24_hour_time() {}
   FMT_CONSTEXPR void on_iso_time() {}
   FMT_CONSTEXPR void on_am_pm() {}
-  FMT_CONSTEXPR void on_duration_value() {}
-  FMT_CONSTEXPR void on_duration_unit() {}
+  FMT_CONSTEXPR void on_utc_offset() {}
+  FMT_CONSTEXPR void on_tz_name() {}
 };
 
-template <typename T, FMT_ENABLE_IF(std::is_integral<T>::value)>
-inline bool isnan(T) {
-  return false;
-}
-template <typename T, FMT_ENABLE_IF(std::is_floating_point<T>::value)>
-inline bool isnan(T value) {
-  return std::isnan(value);
-}
-
-template <typename T, FMT_ENABLE_IF(std::is_integral<T>::value)>
-inline bool isfinite(T) {
-  return true;
-}
-
-// Converts value to int and checks that it's in the range [0, upper).
-template <typename T, FMT_ENABLE_IF(std::is_integral<T>::value)>
-inline int to_nonnegative_int(T value, int upper) {
-  FMT_ASSERT(value >= 0 && to_unsigned(value) <= to_unsigned(upper),
-             "invalid value");
-  (void)upper;
-  return static_cast<int>(value);
+inline const char* tm_wday_full_name(int wday) {
+  static constexpr const char* full_name_list[] = {
+      "Sunday",   "Monday", "Tuesday", "Wednesday",
+      "Thursday", "Friday", "Saturday"};
+  return wday >= 0 && wday <= 6 ? full_name_list[wday] : "?";
 }
-template <typename T, FMT_ENABLE_IF(!std::is_integral<T>::value)>
-inline int to_nonnegative_int(T value, int upper) {
-  FMT_ASSERT(
-      std::isnan(value) || (value >= 0 && value <= static_cast<T>(upper)),
-      "invalid value");
-  (void)upper;
-  return static_cast<int>(value);
+inline const char* tm_wday_short_name(int wday) {
+  static constexpr const char* short_name_list[] = {"Sun", "Mon", "Tue", "Wed",
+                                                    "Thu", "Fri", "Sat"};
+  return wday >= 0 && wday <= 6 ? short_name_list[wday] : "???";
 }
 
-template <typename T, FMT_ENABLE_IF(std::is_integral<T>::value)>
-inline T mod(T x, int y) {
-  return x % static_cast<T>(y);
-}
-template <typename T, FMT_ENABLE_IF(std::is_floating_point<T>::value)>
-inline T mod(T x, int y) {
-  return std::fmod(x, static_cast<T>(y));
+inline const char* tm_mon_full_name(int mon) {
+  static constexpr const char* full_name_list[] = {
+      "January", "February", "March",     "April",   "May",      "June",
+      "July",    "August",   "September", "October", "November", "December"};
+  return mon >= 0 && mon <= 11 ? full_name_list[mon] : "?";
 }
-
-// If T is an integral type, maps T to its unsigned counterpart, otherwise
-// leaves it unchanged (unlike std::make_unsigned).
-template <typename T, bool INTEGRAL = std::is_integral<T>::value>
-struct make_unsigned_or_unchanged {
-  using type = T;
-};
-
-template <typename T> struct make_unsigned_or_unchanged<T, true> {
-  using type = typename std::make_unsigned<T>::type;
-};
-
-#if FMT_SAFE_DURATION_CAST
-// throwing version of safe_duration_cast
-template <typename To, typename FromRep, typename FromPeriod>
-To fmt_safe_duration_cast(std::chrono::duration<FromRep, FromPeriod> from) {
-  int ec;
-  To to = safe_duration_cast::safe_duration_cast<To>(from, ec);
-  if (ec) FMT_THROW(format_error("cannot format duration"));
-  return to;
+inline const char* tm_mon_short_name(int mon) {
+  static constexpr const char* short_name_list[] = {
+      "Jan", "Feb", "Mar", "Apr", "May", "Jun",
+      "Jul", "Aug", "Sep", "Oct", "Nov", "Dec",
+  };
+  return mon >= 0 && mon <= 11 ? short_name_list[mon] : "???";
 }
-#endif
 
-template <typename Rep, typename Period,
-          FMT_ENABLE_IF(std::is_integral<Rep>::value)>
-inline std::chrono::duration<Rep, std::milli> get_milliseconds(
-    std::chrono::duration<Rep, Period> d) {
-  // this may overflow and/or the result may not fit in the
-  // target type.
-#if FMT_SAFE_DURATION_CAST
-  using CommonSecondsType =
-      typename std::common_type<decltype(d), std::chrono::seconds>::type;
-  const auto d_as_common = fmt_safe_duration_cast<CommonSecondsType>(d);
-  const auto d_as_whole_seconds =
-      fmt_safe_duration_cast<std::chrono::seconds>(d_as_common);
-  // this conversion should be nonproblematic
-  const auto diff = d_as_common - d_as_whole_seconds;
-  const auto ms =
-      fmt_safe_duration_cast<std::chrono::duration<Rep, std::milli>>(diff);
-  return ms;
-#else
-  auto s = std::chrono::duration_cast<std::chrono::seconds>(d);
-  return std::chrono::duration_cast<std::chrono::milliseconds>(d - s);
-#endif
-}
+template <typename OutputIt, typename Char> class tm_writer {
+ private:
+  static constexpr int days_per_week = 7;
 
-template <typename Rep, typename Period,
-          FMT_ENABLE_IF(std::is_floating_point<Rep>::value)>
-inline std::chrono::duration<Rep, std::milli> get_milliseconds(
-    std::chrono::duration<Rep, Period> d) {
-  using common_type = typename std::common_type<Rep, std::intmax_t>::type;
-  auto ms = mod(d.count() * static_cast<common_type>(Period::num) /
-                    static_cast<common_type>(Period::den) * 1000,
-                1000);
-  return std::chrono::duration<Rep, std::milli>(static_cast<Rep>(ms));
-}
+  const std::locale& loc_;
+  const bool is_classic_;
+  OutputIt out_;
+  const std::tm& tm_;
 
-template <typename Char, typename Rep, typename OutputIt,
-          FMT_ENABLE_IF(std::is_integral<Rep>::value)>
-OutputIt format_duration_value(OutputIt out, Rep val, int) {
-  return write<Char>(out, val);
-}
+  auto tm_sec() const noexcept -> int {
+    FMT_ASSERT(tm_.tm_sec >= 0 && tm_.tm_sec <= 61, "");
+    return tm_.tm_sec;
+  }
+  auto tm_min() const noexcept -> int {
+    FMT_ASSERT(tm_.tm_min >= 0 && tm_.tm_min <= 59, "");
+    return tm_.tm_min;
+  }
+  auto tm_hour() const noexcept -> int {
+    FMT_ASSERT(tm_.tm_hour >= 0 && tm_.tm_hour <= 23, "");
+    return tm_.tm_hour;
+  }
+  auto tm_mday() const noexcept -> int {
+    FMT_ASSERT(tm_.tm_mday >= 1 && tm_.tm_mday <= 31, "");
+    return tm_.tm_mday;
+  }
+  auto tm_mon() const noexcept -> int {
+    FMT_ASSERT(tm_.tm_mon >= 0 && tm_.tm_mon <= 11, "");
+    return tm_.tm_mon;
+  }
+  auto tm_year() const noexcept -> long long { return 1900ll + tm_.tm_year; }
+  auto tm_wday() const noexcept -> int {
+    FMT_ASSERT(tm_.tm_wday >= 0 && tm_.tm_wday <= 6, "");
+    return tm_.tm_wday;
+  }
+  auto tm_yday() const noexcept -> int {
+    FMT_ASSERT(tm_.tm_yday >= 0 && tm_.tm_yday <= 365, "");
+    return tm_.tm_yday;
+  }
 
-template <typename Char, typename Rep, typename OutputIt,
-          FMT_ENABLE_IF(std::is_floating_point<Rep>::value)>
-OutputIt format_duration_value(OutputIt out, Rep val, int precision) {
-  auto specs = basic_format_specs<Char>();
-  specs.precision = precision;
-  specs.type = precision >= 0 ? presentation_type::fixed_lower
-                              : presentation_type::general_lower;
-  return write<Char>(out, val, specs);
-}
+  auto tm_hour12() const noexcept -> int {
+    const auto h = tm_hour();
+    const auto z = h < 12 ? h : h - 12;
+    return z == 0 ? 12 : z;
+  }
 
-template <typename Char, typename OutputIt>
-OutputIt copy_unit(string_view unit, OutputIt out, Char) {
-  return std::copy(unit.begin(), unit.end(), out);
-}
+  // POSIX and the C Standard are unclear or inconsistent about what %C and %y
+  // do if the year is negative or exceeds 9999. Use the convention that %C
+  // concatenated with %y yields the same output as %Y, and that %Y contains at
+  // least 4 characters, with more only if necessary.
+  auto split_year_lower(long long year) const noexcept -> int {
+    auto l = year % 100;
+    if (l < 0) l = -l;  // l in [0, 99]
+    return static_cast<int>(l);
+  }
 
-template <typename OutputIt>
-OutputIt copy_unit(string_view unit, OutputIt out, wchar_t) {
-  // This works when wchar_t is UTF-32 because units only contain characters
-  // that have the same representation in UTF-16 and UTF-32.
-  utf8_to_utf16 u(unit);
-  return std::copy(u.c_str(), u.c_str() + u.size(), out);
-}
+  // Algorithm:
+  // https://en.wikipedia.org/wiki/ISO_week_date#Calculating_the_week_number_from_a_month_and_day_of_the_month_or_ordinal_date
+  auto iso_year_weeks(long long curr_year) const noexcept -> int {
+    const auto prev_year = curr_year - 1;
+    const auto curr_p =
+        (curr_year + curr_year / 4 - curr_year / 100 + curr_year / 400) %
+        days_per_week;
+    const auto prev_p =
+        (prev_year + prev_year / 4 - prev_year / 100 + prev_year / 400) %
+        days_per_week;
+    return 52 + ((curr_p == 4 || prev_p == 3) ? 1 : 0);
+  }
+  auto iso_week_num(int tm_yday, int tm_wday) const noexcept -> int {
+    return (tm_yday + 11 - (tm_wday == 0 ? days_per_week : tm_wday)) /
+           days_per_week;
+  }
+  auto tm_iso_week_year() const noexcept -> long long {
+    const auto year = tm_year();
+    const auto w = iso_week_num(tm_yday(), tm_wday());
+    if (w < 1) return year - 1;
+    if (w > iso_year_weeks(year)) return year + 1;
+    return year;
+  }
+  auto tm_iso_week_of_year() const noexcept -> int {
+    const auto year = tm_year();
+    const auto w = iso_week_num(tm_yday(), tm_wday());
+    if (w < 1) return iso_year_weeks(year - 1);
+    if (w > iso_year_weeks(year)) return 1;
+    return w;
+  }
 
-template <typename Char, typename Period, typename OutputIt>
-OutputIt format_duration_unit(OutputIt out) {
-  if (const char* unit = get_units<Period>())
-    return copy_unit(string_view(unit), out, Char());
-  *out++ = '[';
-  out = write<Char>(out, Period::num);
-  if (const_check(Period::den != 1)) {
-    *out++ = '/';
-    out = write<Char>(out, Period::den);
+  void write1(int value) {
+    *out_++ = static_cast<char>('0' + to_unsigned(value) % 10);
   }
-  *out++ = ']';
-  *out++ = 's';
-  return out;
-}
-
-template <typename FormatContext, typename OutputIt, typename Rep,
-          typename Period>
-struct chrono_formatter {
-  FormatContext& context;
-  OutputIt out;
-  int precision;
-  bool localized = false;
-  // rep is unsigned to avoid overflow.
-  using rep =
-      conditional_t<std::is_integral<Rep>::value && sizeof(Rep) < sizeof(int),
-                    unsigned, typename make_unsigned_or_unchanged<Rep>::type>;
-  rep val;
-  using seconds = std::chrono::duration<rep>;
-  seconds s;
-  using milliseconds = std::chrono::duration<rep, std::milli>;
-  bool negative;
-
-  using char_type = typename FormatContext::char_type;
-
-  explicit chrono_formatter(FormatContext& ctx, OutputIt o,
-                            std::chrono::duration<Rep, Period> d)
-      : context(ctx),
-        out(o),
-        val(static_cast<rep>(d.count())),
-        negative(false) {
-    if (d.count() < 0) {
-      val = 0 - val;
-      negative = true;
-    }
-
-    // this may overflow and/or the result may not fit in the
-    // target type.
-#if FMT_SAFE_DURATION_CAST
-    // might need checked conversion (rep!=Rep)
-    auto tmpval = std::chrono::duration<rep, Period>(val);
-    s = fmt_safe_duration_cast<seconds>(tmpval);
-#else
-    s = std::chrono::duration_cast<seconds>(
-        std::chrono::duration<rep, Period>(val));
-#endif
+  void write2(int value) {
+    const char* d = digits2(to_unsigned(value) % 100);
+    *out_++ = *d++;
+    *out_++ = *d;
   }
 
-  // returns true if nan or inf, writes to out.
-  bool handle_nan_inf() {
-    if (isfinite(val)) {
-      return false;
-    }
-    if (isnan(val)) {
-      write_nan();
-      return true;
+  void write_year_extended(long long year) {
+    // At least 4 characters.
+    int width = 4;
+    if (year < 0) {
+      *out_++ = '-';
+      year = 0 - year;
+      --width;
     }
-    // must be +-inf
-    if (val > 0) {
-      write_pinf();
+    uint32_or_64_or_128_t<long long> n = to_unsigned(year);
+    const int num_digits = count_digits(n);
+    if (width > num_digits) out_ = std::fill_n(out_, width - num_digits, '0');
+    out_ = format_decimal<Char>(out_, n, num_digits).end;
+  }
+  void write_year(long long year) {
+    if (year >= 0 && year < 10000) {
+      write2(static_cast<int>(year / 100));
+      write2(static_cast<int>(year % 100));
     } else {
-      write_ninf();
+      write_year_extended(year);
     }
-    return true;
   }
 
-  Rep hour() const { return static_cast<Rep>(mod((s.count() / 3600), 24)); }
-
-  Rep hour12() const {
-    Rep hour = static_cast<Rep>(mod((s.count() / 3600), 12));
-    return hour <= 0 ? 12 : hour;
+  void format_localized(char format, char modifier = 0) {
+    out_ = write<Char>(out_, tm_, loc_, format, modifier);
   }
 
-  Rep minute() const { return static_cast<Rep>(mod((s.count() / 60), 60)); }
-  Rep second() const { return static_cast<Rep>(mod(s.count(), 60)); }
+ public:
+  explicit tm_writer(const std::locale& loc, OutputIt out, const std::tm& tm)
+      : loc_(loc),
+        is_classic_(loc_ == std::locale::classic()),
+        out_(out),
+        tm_(tm) {}
 
-  std::tm time() const {
-    auto time = std::tm();
-    time.tm_hour = to_nonnegative_int(hour(), 24);
-    time.tm_min = to_nonnegative_int(minute(), 60);
-    time.tm_sec = to_nonnegative_int(second(), 60);
-    return time;
-  }
+  OutputIt out() const { return out_; }
 
-  void write_sign() {
-    if (negative) {
-      *out++ = '-';
-      negative = false;
-    }
+  FMT_CONSTEXPR void on_text(const Char* begin, const Char* end) {
+    out_ = copy_str<Char>(begin, end, out_);
   }
 
-  void write(Rep value, int width) {
-    write_sign();
-    if (isnan(value)) return write_nan();
-    uint32_or_64_or_128_t<int> n =
-        to_unsigned(to_nonnegative_int(value, max_value<int>()));
-    int num_digits = detail::count_digits(n);
-    if (width > num_digits) out = std::fill_n(out, width - num_digits, '0');
-    out = format_decimal<char_type>(out, n, num_digits).end;
+  void on_abbr_weekday() {
+    if (is_classic_)
+      out_ = write(out_, tm_wday_short_name(tm_wday()));
+    else
+      format_localized('a');
   }
-
-  void write_nan() { std::copy_n("nan", 3, out); }
-  void write_pinf() { std::copy_n("inf", 3, out); }
-  void write_ninf() { std::copy_n("-inf", 4, out); }
-
-  void format_localized(const tm& time, char format, char modifier = 0) {
-    if (isnan(val)) return write_nan();
-    const auto& loc = localized ? context.locale().template get<std::locale>()
-                                : std::locale::classic();
-    out = detail::write<char_type>(out, time, loc, format, modifier);
+  void on_full_weekday() {
+    if (is_classic_)
+      out_ = write(out_, tm_wday_full_name(tm_wday()));
+    else
+      format_localized('A');
   }
-
-  void on_text(const char_type* begin, const char_type* end) {
-    std::copy(begin, end, out);
+  void on_dec0_weekday(numeric_system ns) {
+    if (ns != numeric_system::standard) return format_localized('w', 'O');
+    write1(tm_wday());
   }
-
-  // These are not implemented because durations don't have date information.
-  void on_abbr_weekday() {}
-  void on_full_weekday() {}
-  void on_dec0_weekday(numeric_system) {}
-  void on_dec1_weekday(numeric_system) {}
-  void on_abbr_month() {}
-  void on_full_month() {}
-  void on_datetime(numeric_system) {}
-  void on_loc_date(numeric_system) {}
-  void on_loc_time(numeric_system) {}
-  void on_us_date() {}
-  void on_iso_date() {}
-  void on_utc_offset() {}
-  void on_tz_name() {}
-  void on_year(numeric_system) {}
-  void on_short_year(numeric_system) {}
-  void on_offset_year() {}
-  void on_century(numeric_system) {}
-  void on_iso_week_based_year() {}
-  void on_iso_week_based_short_year() {}
-  void on_dec_month(numeric_system) {}
-  void on_dec0_week_of_year(numeric_system) {}
-  void on_dec1_week_of_year(numeric_system) {}
-  void on_iso_week_of_year(numeric_system) {}
-  void on_day_of_year() {}
-  void on_day_of_month(numeric_system) {}
-  void on_day_of_month_space(numeric_system) {}
-
-  void on_24_hour(numeric_system ns) {
-    if (handle_nan_inf()) return;
-
-    if (ns == numeric_system::standard) return write(hour(), 2);
-    auto time = tm();
-    time.tm_hour = to_nonnegative_int(hour(), 24);
-    format_localized(time, 'H', 'O');
+  void on_dec1_weekday(numeric_system ns) {
+    if (ns != numeric_system::standard) return format_localized('u', 'O');
+    auto wday = tm_wday();
+    write1(wday == 0 ? days_per_week : wday);
   }
 
-  void on_12_hour(numeric_system ns) {
-    if (handle_nan_inf()) return;
-
-    if (ns == numeric_system::standard) return write(hour12(), 2);
-    auto time = tm();
-    time.tm_hour = to_nonnegative_int(hour12(), 12);
-    format_localized(time, 'I', 'O');
+  void on_abbr_month() {
+    if (is_classic_)
+      out_ = write(out_, tm_mon_short_name(tm_mon()));
+    else
+      format_localized('b');
   }
-
-  void on_minute(numeric_system ns) {
-    if (handle_nan_inf()) return;
-
-    if (ns == numeric_system::standard) return write(minute(), 2);
-    auto time = tm();
-    time.tm_min = to_nonnegative_int(minute(), 60);
-    format_localized(time, 'M', 'O');
+  void on_full_month() {
+    if (is_classic_)
+      out_ = write(out_, tm_mon_full_name(tm_mon()));
+    else
+      format_localized('B');
   }
 
-  void on_second(numeric_system ns) {
-    if (handle_nan_inf()) return;
-
-    if (ns == numeric_system::standard) {
-      write(second(), 2);
-#if FMT_SAFE_DURATION_CAST
-      // convert rep->Rep
-      using duration_rep = std::chrono::duration<rep, Period>;
-      using duration_Rep = std::chrono::duration<Rep, Period>;
-      auto tmpval = fmt_safe_duration_cast<duration_Rep>(duration_rep{val});
-#else
-      auto tmpval = std::chrono::duration<Rep, Period>(val);
-#endif
-      auto ms = get_milliseconds(tmpval);
-      if (ms != std::chrono::milliseconds(0)) {
-        *out++ = '.';
-        write(ms.count(), 3);
-      }
-      return;
+  void on_datetime(numeric_system ns) {
+    if (is_classic_) {
+      on_abbr_weekday();
+      *out_++ = ' ';
+      on_abbr_month();
+      *out_++ = ' ';
+      on_day_of_month_space(numeric_system::standard);
+      *out_++ = ' ';
+      on_iso_time();
+      *out_++ = ' ';
+      on_year(numeric_system::standard);
+    } else {
+      format_localized('c', ns == numeric_system::standard ? '\0' : 'E');
     }
-    auto time = tm();
-    time.tm_sec = to_nonnegative_int(second(), 60);
-    format_localized(time, 'S', 'O');
   }
-
-  void on_12_hour_time() {
-    if (handle_nan_inf()) return;
-    format_localized(time(), 'r');
+  void on_loc_date(numeric_system ns) {
+    if (is_classic_)
+      on_us_date();
+    else
+      format_localized('x', ns == numeric_system::standard ? '\0' : 'E');
   }
-
-  void on_24_hour_time() {
-    if (handle_nan_inf()) {
-      *out++ = ':';
-      handle_nan_inf();
-      return;
+  void on_loc_time(numeric_system ns) {
+    if (is_classic_)
+      on_iso_time();
+    else
+      format_localized('X', ns == numeric_system::standard ? '\0' : 'E');
+  }
+  void on_us_date() {
+    char buf[8];
+    write_digit2_separated(buf, to_unsigned(tm_mon() + 1),
+                           to_unsigned(tm_mday()),
+                           to_unsigned(split_year_lower(tm_year())), '/');
+    out_ = copy_str<Char>(std::begin(buf), std::end(buf), out_);
+  }
+  void on_iso_date() {
+    auto year = tm_year();
+    char buf[10];
+    size_t offset = 0;
+    if (year >= 0 && year < 10000) {
+      copy2(buf, digits2(to_unsigned(year / 100)));
+    } else {
+      offset = 4;
+      write_year_extended(year);
+      year = 0;
     }
-
-    write(hour(), 2);
-    *out++ = ':';
-    write(minute(), 2);
+    write_digit2_separated(buf + 2, static_cast<unsigned>(year % 100),
+                           to_unsigned(tm_mon() + 1), to_unsigned(tm_mday()),
+                           '-');
+    out_ = copy_str<Char>(std::begin(buf) + offset, std::end(buf), out_);
   }
 
-  void on_iso_time() {
-    on_24_hour_time();
-    *out++ = ':';
-    if (handle_nan_inf()) return;
-    write(second(), 2);
-  }
+  void on_utc_offset() { format_localized('z'); }
+  void on_tz_name() { format_localized('Z'); }
 
-  void on_am_pm() {
-    if (handle_nan_inf()) return;
-    format_localized(time(), 'p');
+  void on_year(numeric_system ns) {
+    if (ns != numeric_system::standard) return format_localized('Y', 'E');
+    write_year(tm_year());
   }
-
-  void on_duration_value() {
-    if (handle_nan_inf()) return;
-    write_sign();
-    out = format_duration_value<char_type>(out, val, precision);
+  void on_short_year(numeric_system ns) {
+    if (ns != numeric_system::standard) return format_localized('y', 'O');
+    write2(split_year_lower(tm_year()));
   }
+  void on_offset_year() { format_localized('y', 'E'); }
 
-  void on_duration_unit() {
-    out = format_duration_unit<char_type, Period>(out);
+  void on_century(numeric_system ns) {
+    if (ns != numeric_system::standard) return format_localized('C', 'E');
+    auto year = tm_year();
+    auto upper = year / 100;
+    if (year >= -99 && year < 0) {
+      // Zero upper on negative year.
+      *out_++ = '-';
+      *out_++ = '0';
+    } else if (upper >= 0 && upper < 100) {
+      write2(static_cast<int>(upper));
+    } else {
+      out_ = write<Char>(out_, upper);
+    }
   }
-};
 
-FMT_END_DETAIL_NAMESPACE
+  void on_dec_month(numeric_system ns) {
+    if (ns != numeric_system::standard) return format_localized('m', 'O');
+    write2(tm_mon() + 1);
+  }
 
-#if defined(__cpp_lib_chrono) && __cpp_lib_chrono >= 201907
-using weekday = std::chrono::weekday;
-#else
-// A fallback version of weekday.
-class weekday {
- private:
-  unsigned char value;
+  void on_dec0_week_of_year(numeric_system ns) {
+    if (ns != numeric_system::standard) return format_localized('U', 'O');
+    write2((tm_yday() + days_per_week - tm_wday()) / days_per_week);
+  }
+  void on_dec1_week_of_year(numeric_system ns) {
+    if (ns != numeric_system::standard) return format_localized('W', 'O');
+    auto wday = tm_wday();
+    write2((tm_yday() + days_per_week -
+            (wday == 0 ? (days_per_week - 1) : (wday - 1))) /
+           days_per_week);
+  }
+  void on_iso_week_of_year(numeric_system ns) {
+    if (ns != numeric_system::standard) return format_localized('V', 'O');
+    write2(tm_iso_week_of_year());
+  }
 
- public:
-  weekday() = default;
-  explicit constexpr weekday(unsigned wd) noexcept
-      : value(static_cast<unsigned char>(wd != 7 ? wd : 0)) {}
-  constexpr unsigned c_encoding() const noexcept { return value; }
-};
+  void on_iso_week_based_year() { write_year(tm_iso_week_year()); }
+  void on_iso_week_based_short_year() {
+    write2(split_year_lower(tm_iso_week_year()));
+  }
 
-class year_month_day {};
-#endif
+  void on_day_of_year() {
+    auto yday = tm_yday() + 1;
+    write1(yday / 100);
+    write2(yday % 100);
+  }
+  void on_day_of_month(numeric_system ns) {
+    if (ns != numeric_system::standard) return format_localized('d', 'O');
+    write2(tm_mday());
+  }
+  void on_day_of_month_space(numeric_system ns) {
+    if (ns != numeric_system::standard) return format_localized('e', 'O');
+    auto mday = to_unsigned(tm_mday()) % 100;
+    const char* d2 = digits2(mday);
+    *out_++ = mday < 10 ? ' ' : d2[0];
+    *out_++ = d2[1];
+  }
 
-// A rudimentary weekday formatter.
-template <typename Char> struct formatter<weekday, Char> {
- private:
-  bool localized = false;
+  void on_24_hour(numeric_system ns) {
+    if (ns != numeric_system::standard) return format_localized('H', 'O');
+    write2(tm_hour());
+  }
+  void on_12_hour(numeric_system ns) {
+    if (ns != numeric_system::standard) return format_localized('I', 'O');
+    write2(tm_hour12());
+  }
+  void on_minute(numeric_system ns) {
+    if (ns != numeric_system::standard) return format_localized('M', 'O');
+    write2(tm_min());
+  }
+  void on_second(numeric_system ns) {
+    if (ns != numeric_system::standard) return format_localized('S', 'O');
+    write2(tm_sec());
+  }
 
- public:
-  FMT_CONSTEXPR auto parse(basic_format_parse_context<Char>& ctx)
-      -> decltype(ctx.begin()) {
-    auto begin = ctx.begin(), end = ctx.end();
-    if (begin != end && *begin == 'L') {
-      ++begin;
-      localized = true;
+  void on_12_hour_time() {
+    if (is_classic_) {
+      char buf[8];
+      write_digit2_separated(buf, to_unsigned(tm_hour12()),
+                             to_unsigned(tm_min()), to_unsigned(tm_sec()), ':');
+      out_ = copy_str<Char>(std::begin(buf), std::end(buf), out_);
+      *out_++ = ' ';
+      on_am_pm();
+    } else {
+      format_localized('r');
     }
-    return begin;
+  }
+  void on_24_hour_time() {
+    write2(tm_hour());
+    *out_++ = ':';
+    write2(tm_min());
+  }
+  void on_iso_time() {
+    char buf[8];
+    write_digit2_separated(buf, to_unsigned(tm_hour()), to_unsigned(tm_min()),
+                           to_unsigned(tm_sec()), ':');
+    out_ = copy_str<Char>(std::begin(buf), std::end(buf), out_);
   }
 
-  template <typename FormatContext>
-  auto format(weekday wd, FormatContext& ctx) const -> decltype(ctx.out()) {
-    auto time = std::tm();
-    time.tm_wday = static_cast<int>(wd.c_encoding());
-    const auto& loc = localized ? ctx.locale().template get<std::locale>()
-                                : std::locale::classic();
-    return detail::write<Char>(ctx.out(), time, loc, 'a');
+  void on_am_pm() {
+    if (is_classic_) {
+      *out_++ = tm_hour() < 12 ? 'A' : 'P';
+      *out_++ = 'M';
+    } else {
+      format_localized('p');
+    }
   }
-};
 
-template <typename Rep, typename Period, typename Char>
-struct formatter<std::chrono::duration<Rep, Period>, Char> {
- private:
-  basic_format_specs<Char> specs;
-  int precision = -1;
-  using arg_ref_type = detail::arg_ref<Char>;
-  arg_ref_type width_ref;
-  arg_ref_type precision_ref;
-  bool localized = false;
-  basic_string_view<Char> format_str;
-  using duration = std::chrono::duration<Rep, Period>;
+  // These apply to chrono durations but not tm.
+  void on_duration_value() {}
+  void on_duration_unit() {}
+};
 
-  struct spec_handler {
-    formatter& f;
-    basic_format_parse_context<Char>& context;
-    basic_string_view<Char> format_str;
+struct chrono_format_checker : null_chrono_spec_handler<chrono_format_checker> {
+  FMT_NORETURN void unsupported() { FMT_THROW(format_error("no date")); }
 
-    template <typename Id> FMT_CONSTEXPR arg_ref_type make_arg_ref(Id arg_id) {
-      context.check_arg_id(arg_id);
-      return arg_ref_type(arg_id);
-    }
+  template <typename Char>
+  FMT_CONSTEXPR void on_text(const Char*, const Char*) {}
+  FMT_CONSTEXPR void on_24_hour(numeric_system) {}
+  FMT_CONSTEXPR void on_12_hour(numeric_system) {}
+  FMT_CONSTEXPR void on_minute(numeric_system) {}
+  FMT_CONSTEXPR void on_second(numeric_system) {}
+  FMT_CONSTEXPR void on_12_hour_time() {}
+  FMT_CONSTEXPR void on_24_hour_time() {}
+  FMT_CONSTEXPR void on_iso_time() {}
+  FMT_CONSTEXPR void on_am_pm() {}
+  FMT_CONSTEXPR void on_duration_value() {}
+  FMT_CONSTEXPR void on_duration_unit() {}
+};
 
-    FMT_CONSTEXPR arg_ref_type make_arg_ref(basic_string_view<Char> arg_id) {
-      context.check_arg_id(arg_id);
-      return arg_ref_type(arg_id);
-    }
+template <typename T, FMT_ENABLE_IF(std::is_integral<T>::value)>
+inline bool isnan(T) {
+  return false;
+}
+template <typename T, FMT_ENABLE_IF(std::is_floating_point<T>::value)>
+inline bool isnan(T value) {
+  return std::isnan(value);
+}
 
-    FMT_CONSTEXPR arg_ref_type make_arg_ref(detail::auto_id) {
-      return arg_ref_type(context.next_arg_id());
-    }
+template <typename T, FMT_ENABLE_IF(std::is_integral<T>::value)>
+inline bool isfinite(T) {
+  return true;
+}
 
-    void on_error(const char* msg) { FMT_THROW(format_error(msg)); }
-    FMT_CONSTEXPR void on_fill(basic_string_view<Char> fill) {
-      f.specs.fill = fill;
-    }
-    FMT_CONSTEXPR void on_align(align_t align) { f.specs.align = align; }
-    FMT_CONSTEXPR void on_width(int width) { f.specs.width = width; }
-    FMT_CONSTEXPR void on_precision(int _precision) {
-      f.precision = _precision;
-    }
-    FMT_CONSTEXPR void end_precision() {}
-
-    template <typename Id> FMT_CONSTEXPR void on_dynamic_width(Id arg_id) {
-      f.width_ref = make_arg_ref(arg_id);
-    }
-
-    template <typename Id> FMT_CONSTEXPR void on_dynamic_precision(Id arg_id) {
-      f.precision_ref = make_arg_ref(arg_id);
-    }
-  };
-
-  using iterator = typename basic_format_parse_context<Char>::iterator;
-  struct parse_range {
-    iterator begin;
-    iterator end;
-  };
-
-  FMT_CONSTEXPR parse_range do_parse(basic_format_parse_context<Char>& ctx) {
-    auto begin = ctx.begin(), end = ctx.end();
-    if (begin == end || *begin == '}') return {begin, begin};
-    spec_handler handler{*this, ctx, format_str};
-    begin = detail::parse_align(begin, end, handler);
-    if (begin == end) return {begin, begin};
-    begin = detail::parse_width(begin, end, handler);
-    if (begin == end) return {begin, begin};
-    if (*begin == '.') {
-      if (std::is_floating_point<Rep>::value)
-        begin = detail::parse_precision(begin, end, handler);
-      else
-        handler.on_error("precision not allowed for this argument type");
-    }
-    if (begin != end && *begin == 'L') {
-      ++begin;
-      localized = true;
-    }
-    end = detail::parse_chrono_format(begin, end,
-                                      detail::chrono_format_checker());
-    return {begin, end};
-  }
+// Converts value to int and checks that it's in the range [0, upper).
+template <typename T, FMT_ENABLE_IF(std::is_integral<T>::value)>
+inline int to_nonnegative_int(T value, int upper) {
+  FMT_ASSERT(value >= 0 && to_unsigned(value) <= to_unsigned(upper),
+             "invalid value");
+  (void)upper;
+  return static_cast<int>(value);
+}
+template <typename T, FMT_ENABLE_IF(!std::is_integral<T>::value)>
+inline int to_nonnegative_int(T value, int upper) {
+  FMT_ASSERT(
+      std::isnan(value) || (value >= 0 && value <= static_cast<T>(upper)),
+      "invalid value");
+  (void)upper;
+  return static_cast<int>(value);
+}
 
- public:
-  FMT_CONSTEXPR auto parse(basic_format_parse_context<Char>& ctx)
-      -> decltype(ctx.begin()) {
-    auto range = do_parse(ctx);
-    format_str = basic_string_view<Char>(
-        &*range.begin, detail::to_unsigned(range.end - range.begin));
-    return range.end;
-  }
+template <typename T, FMT_ENABLE_IF(std::is_integral<T>::value)>
+inline T mod(T x, int y) {
+  return x % static_cast<T>(y);
+}
+template <typename T, FMT_ENABLE_IF(std::is_floating_point<T>::value)>
+inline T mod(T x, int y) {
+  return std::fmod(x, static_cast<T>(y));
+}
 
-  template <typename FormatContext>
-  auto format(const duration& d, FormatContext& ctx) const
-      -> decltype(ctx.out()) {
-    auto specs_copy = specs;
-    auto precision_copy = precision;
-    auto begin = format_str.begin(), end = format_str.end();
-    // As a possible future optimization, we could avoid extra copying if width
-    // is not specified.
-    basic_memory_buffer<Char> buf;
-    auto out = std::back_inserter(buf);
-    detail::handle_dynamic_spec<detail::width_checker>(specs_copy.width,
-                                                       width_ref, ctx);
-    detail::handle_dynamic_spec<detail::precision_checker>(precision_copy,
-                                                           precision_ref, ctx);
-    if (begin == end || *begin == '}') {
-      out = detail::format_duration_value<Char>(out, d.count(), precision_copy);
-      detail::format_duration_unit<Char, Period>(out);
-    } else {
-      detail::chrono_formatter<FormatContext, decltype(out), Rep, Period> f(
-          ctx, out, d);
-      f.precision = precision_copy;
-      f.localized = localized;
-      detail::parse_chrono_format(begin, end, f);
-    }
-    return detail::write(
-        ctx.out(), basic_string_view<Char>(buf.data(), buf.size()), specs_copy);
-  }
+// If T is an integral type, maps T to its unsigned counterpart, otherwise
+// leaves it unchanged (unlike std::make_unsigned).
+template <typename T, bool INTEGRAL = std::is_integral<T>::value>
+struct make_unsigned_or_unchanged {
+  using type = T;
 };
 
-FMT_BEGIN_DETAIL_NAMESPACE
-
-struct tm_format_checker : null_chrono_spec_handler<tm_format_checker> {
-  FMT_NORETURN void unsupported() { FMT_THROW(format_error("no format")); }
-
-  template <typename Char>
-  FMT_CONSTEXPR void on_text(const Char*, const Char*) {}
-  FMT_CONSTEXPR void on_year(numeric_system) {}
-  FMT_CONSTEXPR void on_short_year(numeric_system) {}
-  FMT_CONSTEXPR void on_offset_year() {}
-  FMT_CONSTEXPR void on_century(numeric_system) {}
-  FMT_CONSTEXPR void on_iso_week_based_year() {}
-  FMT_CONSTEXPR void on_iso_week_based_short_year() {}
-  FMT_CONSTEXPR void on_abbr_weekday() {}
-  FMT_CONSTEXPR void on_full_weekday() {}
-  FMT_CONSTEXPR void on_dec0_weekday(numeric_system) {}
-  FMT_CONSTEXPR void on_dec1_weekday(numeric_system) {}
-  FMT_CONSTEXPR void on_abbr_month() {}
-  FMT_CONSTEXPR void on_full_month() {}
-  FMT_CONSTEXPR void on_dec_month(numeric_system) {}
-  FMT_CONSTEXPR void on_dec0_week_of_year(numeric_system) {}
-  FMT_CONSTEXPR void on_dec1_week_of_year(numeric_system) {}
-  FMT_CONSTEXPR void on_iso_week_of_year(numeric_system) {}
-  FMT_CONSTEXPR void on_day_of_year() {}
-  FMT_CONSTEXPR void on_day_of_month(numeric_system) {}
-  FMT_CONSTEXPR void on_day_of_month_space(numeric_system) {}
-  FMT_CONSTEXPR void on_24_hour(numeric_system) {}
-  FMT_CONSTEXPR void on_12_hour(numeric_system) {}
-  FMT_CONSTEXPR void on_minute(numeric_system) {}
-  FMT_CONSTEXPR void on_second(numeric_system) {}
-  FMT_CONSTEXPR void on_datetime(numeric_system) {}
-  FMT_CONSTEXPR void on_loc_date(numeric_system) {}
-  FMT_CONSTEXPR void on_loc_time(numeric_system) {}
-  FMT_CONSTEXPR void on_us_date() {}
-  FMT_CONSTEXPR void on_iso_date() {}
-  FMT_CONSTEXPR void on_12_hour_time() {}
-  FMT_CONSTEXPR void on_24_hour_time() {}
-  FMT_CONSTEXPR void on_iso_time() {}
-  FMT_CONSTEXPR void on_am_pm() {}
-  FMT_CONSTEXPR void on_utc_offset() {}
-  FMT_CONSTEXPR void on_tz_name() {}
+template <typename T> struct make_unsigned_or_unchanged<T, true> {
+  using type = typename std::make_unsigned<T>::type;
 };
 
-inline const char* tm_wday_full_name(int wday) {
-  static constexpr const char* full_name_list[] = {
-      "Sunday",   "Monday", "Tuesday", "Wednesday",
-      "Thursday", "Friday", "Saturday"};
-  return wday >= 0 && wday <= 6 ? full_name_list[wday] : "?";
+#if FMT_SAFE_DURATION_CAST
+// throwing version of safe_duration_cast
+template <typename To, typename FromRep, typename FromPeriod>
+To fmt_safe_duration_cast(std::chrono::duration<FromRep, FromPeriod> from) {
+  int ec;
+  To to = safe_duration_cast::safe_duration_cast<To>(from, ec);
+  if (ec) FMT_THROW(format_error("cannot format duration"));
+  return to;
 }
-inline const char* tm_wday_short_name(int wday) {
-  static constexpr const char* short_name_list[] = {"Sun", "Mon", "Tue", "Wed",
-                                                    "Thu", "Fri", "Sat"};
-  return wday >= 0 && wday <= 6 ? short_name_list[wday] : "???";
+#endif
+
+template <typename Rep, typename Period,
+          FMT_ENABLE_IF(std::is_integral<Rep>::value)>
+inline std::chrono::duration<Rep, std::milli> get_milliseconds(
+    std::chrono::duration<Rep, Period> d) {
+  // this may overflow and/or the result may not fit in the
+  // target type.
+#if FMT_SAFE_DURATION_CAST
+  using CommonSecondsType =
+      typename std::common_type<decltype(d), std::chrono::seconds>::type;
+  const auto d_as_common = fmt_safe_duration_cast<CommonSecondsType>(d);
+  const auto d_as_whole_seconds =
+      fmt_safe_duration_cast<std::chrono::seconds>(d_as_common);
+  // this conversion should be nonproblematic
+  const auto diff = d_as_common - d_as_whole_seconds;
+  const auto ms =
+      fmt_safe_duration_cast<std::chrono::duration<Rep, std::milli>>(diff);
+  return ms;
+#else
+  auto s = std::chrono::duration_cast<std::chrono::seconds>(d);
+  return std::chrono::duration_cast<std::chrono::milliseconds>(d - s);
+#endif
 }
 
-inline const char* tm_mon_full_name(int mon) {
-  static constexpr const char* full_name_list[] = {
-      "January", "February", "March",     "April",   "May",      "June",
-      "July",    "August",   "September", "October", "November", "December"};
-  return mon >= 0 && mon <= 11 ? full_name_list[mon] : "?";
+template <typename Rep, typename Period,
+          FMT_ENABLE_IF(std::is_floating_point<Rep>::value)>
+inline std::chrono::duration<Rep, std::milli> get_milliseconds(
+    std::chrono::duration<Rep, Period> d) {
+  using common_type = typename std::common_type<Rep, std::intmax_t>::type;
+  auto ms = mod(d.count() * static_cast<common_type>(Period::num) /
+                    static_cast<common_type>(Period::den) * 1000,
+                1000);
+  return std::chrono::duration<Rep, std::milli>(static_cast<Rep>(ms));
 }
-inline const char* tm_mon_short_name(int mon) {
-  static constexpr const char* short_name_list[] = {
-      "Jan", "Feb", "Mar", "Apr", "May", "Jun",
-      "Jul", "Aug", "Sep", "Oct", "Nov", "Dec",
-  };
-  return mon >= 0 && mon <= 11 ? short_name_list[mon] : "???";
+
+template <typename Char, typename Rep, typename OutputIt,
+          FMT_ENABLE_IF(std::is_integral<Rep>::value)>
+OutputIt format_duration_value(OutputIt out, Rep val, int) {
+  return write<Char>(out, val);
 }
 
-template <typename OutputIt, typename Char> class tm_writer {
- private:
-  static constexpr int days_per_week = 7;
+template <typename Char, typename Rep, typename OutputIt,
+          FMT_ENABLE_IF(std::is_floating_point<Rep>::value)>
+OutputIt format_duration_value(OutputIt out, Rep val, int precision) {
+  auto specs = basic_format_specs<Char>();
+  specs.precision = precision;
+  specs.type = precision >= 0 ? presentation_type::fixed_lower
+                              : presentation_type::general_lower;
+  return write<Char>(out, val, specs);
+}
 
-  const std::locale& loc_;
-  const bool is_classic_;
-  OutputIt out_;
-  const std::tm& tm_;
+template <typename Char, typename OutputIt>
+OutputIt copy_unit(string_view unit, OutputIt out, Char) {
+  return std::copy(unit.begin(), unit.end(), out);
+}
 
-  auto tm_sec() const noexcept -> int {
-    FMT_ASSERT(tm_.tm_sec >= 0 && tm_.tm_sec <= 61, "");
-    return tm_.tm_sec;
-  }
-  auto tm_min() const noexcept -> int {
-    FMT_ASSERT(tm_.tm_min >= 0 && tm_.tm_min <= 59, "");
-    return tm_.tm_min;
-  }
-  auto tm_hour() const noexcept -> int {
-    FMT_ASSERT(tm_.tm_hour >= 0 && tm_.tm_hour <= 23, "");
-    return tm_.tm_hour;
-  }
-  auto tm_mday() const noexcept -> int {
-    FMT_ASSERT(tm_.tm_mday >= 1 && tm_.tm_mday <= 31, "");
-    return tm_.tm_mday;
-  }
-  auto tm_mon() const noexcept -> int {
-    FMT_ASSERT(tm_.tm_mon >= 0 && tm_.tm_mon <= 11, "");
-    return tm_.tm_mon;
-  }
-  auto tm_year() const noexcept -> long long { return 1900ll + tm_.tm_year; }
-  auto tm_wday() const noexcept -> int {
-    FMT_ASSERT(tm_.tm_wday >= 0 && tm_.tm_wday <= 6, "");
-    return tm_.tm_wday;
-  }
-  auto tm_yday() const noexcept -> int {
-    FMT_ASSERT(tm_.tm_yday >= 0 && tm_.tm_yday <= 365, "");
-    return tm_.tm_yday;
-  }
+template <typename OutputIt>
+OutputIt copy_unit(string_view unit, OutputIt out, wchar_t) {
+  // This works when wchar_t is UTF-32 because units only contain characters
+  // that have the same representation in UTF-16 and UTF-32.
+  utf8_to_utf16 u(unit);
+  return std::copy(u.c_str(), u.c_str() + u.size(), out);
+}
 
-  auto tm_hour12() const noexcept -> int {
-    const auto h = tm_hour();
-    const auto z = h < 12 ? h : h - 12;
-    return z == 0 ? 12 : z;
+template <typename Char, typename Period, typename OutputIt>
+OutputIt format_duration_unit(OutputIt out) {
+  if (const char* unit = get_units<Period>())
+    return copy_unit(string_view(unit), out, Char());
+  *out++ = '[';
+  out = write<Char>(out, Period::num);
+  if (const_check(Period::den != 1)) {
+    *out++ = '/';
+    out = write<Char>(out, Period::den);
   }
+  *out++ = ']';
+  *out++ = 's';
+  return out;
+}
 
-  // POSIX and the C Standard are unclear or inconsistent about what %C and %y
-  // do if the year is negative or exceeds 9999. Use the convention that %C
-  // concatenated with %y yields the same output as %Y, and that %Y contains at
-  // least 4 characters, with more only if necessary.
-  auto split_year_lower(long long year) const noexcept -> int {
-    auto l = year % 100;
-    if (l < 0) l = -l;  // l in [0, 99]
-    return static_cast<int>(l);
-  }
+template <typename FormatContext, typename OutputIt, typename Rep,
+          typename Period>
+struct chrono_formatter {
+  FormatContext& context;
+  OutputIt out;
+  int precision;
+  bool localized = false;
+  // rep is unsigned to avoid overflow.
+  using rep =
+      conditional_t<std::is_integral<Rep>::value && sizeof(Rep) < sizeof(int),
+                    unsigned, typename make_unsigned_or_unchanged<Rep>::type>;
+  rep val;
+  using seconds = std::chrono::duration<rep>;
+  seconds s;
+  using milliseconds = std::chrono::duration<rep, std::milli>;
+  bool negative;
 
-  // Algorithm:
-  // https://en.wikipedia.org/wiki/ISO_week_date#Calculating_the_week_number_from_a_month_and_day_of_the_month_or_ordinal_date
-  auto iso_year_weeks(long long curr_year) const noexcept -> int {
-    const auto prev_year = curr_year - 1;
-    const auto curr_p =
-        (curr_year + curr_year / 4 - curr_year / 100 + curr_year / 400) %
-        days_per_week;
-    const auto prev_p =
-        (prev_year + prev_year / 4 - prev_year / 100 + prev_year / 400) %
-        days_per_week;
-    return 52 + ((curr_p == 4 || prev_p == 3) ? 1 : 0);
-  }
-  auto iso_week_num(int tm_yday, int tm_wday) const noexcept -> int {
-    return (tm_yday + 11 - (tm_wday == 0 ? days_per_week : tm_wday)) /
-           days_per_week;
-  }
-  auto tm_iso_week_year() const noexcept -> long long {
-    const auto year = tm_year();
-    const auto w = iso_week_num(tm_yday(), tm_wday());
-    if (w < 1) return year - 1;
-    if (w > iso_year_weeks(year)) return year + 1;
-    return year;
-  }
-  auto tm_iso_week_of_year() const noexcept -> int {
-    const auto year = tm_year();
-    const auto w = iso_week_num(tm_yday(), tm_wday());
-    if (w < 1) return iso_year_weeks(year - 1);
-    if (w > iso_year_weeks(year)) return 1;
-    return w;
-  }
+  using char_type = typename FormatContext::char_type;
 
-  void write1(int value) {
-    *out_++ = static_cast<char>('0' + to_unsigned(value) % 10);
-  }
-  void write2(int value) {
-    const char* d = digits2(to_unsigned(value) % 100);
-    *out_++ = *d++;
-    *out_++ = *d;
+  explicit chrono_formatter(FormatContext& ctx, OutputIt o,
+                            std::chrono::duration<Rep, Period> d)
+      : context(ctx),
+        out(o),
+        val(static_cast<rep>(d.count())),
+        negative(false) {
+    if (d.count() < 0) {
+      val = 0 - val;
+      negative = true;
+    }
+
+    // this may overflow and/or the result may not fit in the
+    // target type.
+#if FMT_SAFE_DURATION_CAST
+    // might need checked conversion (rep!=Rep)
+    auto tmpval = std::chrono::duration<rep, Period>(val);
+    s = fmt_safe_duration_cast<seconds>(tmpval);
+#else
+    s = std::chrono::duration_cast<seconds>(
+        std::chrono::duration<rep, Period>(val));
+#endif
   }
 
-  void write_year_extended(long long year) {
-    // At least 4 characters.
-    int width = 4;
-    if (year < 0) {
-      *out_++ = '-';
-      year = 0 - year;
-      --width;
+  // returns true if nan or inf, writes to out.
+  bool handle_nan_inf() {
+    if (isfinite(val)) {
+      return false;
     }
-    uint32_or_64_or_128_t<long long> n = to_unsigned(year);
-    const int num_digits = count_digits(n);
-    if (width > num_digits) out_ = std::fill_n(out_, width - num_digits, '0');
-    out_ = format_decimal<Char>(out_, n, num_digits).end;
-  }
-  void write_year(long long year) {
-    if (year >= 0 && year < 10000) {
-      write2(static_cast<int>(year / 100));
-      write2(static_cast<int>(year % 100));
+    if (isnan(val)) {
+      write_nan();
+      return true;
+    }
+    // must be +-inf
+    if (val > 0) {
+      write_pinf();
     } else {
-      write_year_extended(year);
+      write_ninf();
     }
+    return true;
   }
 
-  void format_localized(char format, char modifier = 0) {
-    out_ = write<Char>(out_, tm_, loc_, format, modifier);
-  }
+  Rep hour() const { return static_cast<Rep>(mod((s.count() / 3600), 24)); }
 
- public:
-  explicit tm_writer(const std::locale& loc, OutputIt out, const std::tm& tm)
-      : loc_(loc),
-        is_classic_(loc_ == std::locale::classic()),
-        out_(out),
-        tm_(tm) {}
+  Rep hour12() const {
+    Rep hour = static_cast<Rep>(mod((s.count() / 3600), 12));
+    return hour <= 0 ? 12 : hour;
+  }
 
-  OutputIt out() const { return out_; }
+  Rep minute() const { return static_cast<Rep>(mod((s.count() / 60), 60)); }
+  Rep second() const { return static_cast<Rep>(mod(s.count(), 60)); }
 
-  FMT_CONSTEXPR void on_text(const Char* begin, const Char* end) {
-    out_ = copy_str<Char>(begin, end, out_);
+  std::tm time() const {
+    auto time = std::tm();
+    time.tm_hour = to_nonnegative_int(hour(), 24);
+    time.tm_min = to_nonnegative_int(minute(), 60);
+    time.tm_sec = to_nonnegative_int(second(), 60);
+    return time;
   }
 
-  void on_abbr_weekday() {
-    if (is_classic_)
-      out_ = write(out_, tm_wday_short_name(tm_wday()));
-    else
-      format_localized('a');
-  }
-  void on_full_weekday() {
-    if (is_classic_)
-      out_ = write(out_, tm_wday_full_name(tm_wday()));
-    else
-      format_localized('A');
-  }
-  void on_dec0_weekday(numeric_system ns) {
-    if (ns != numeric_system::standard) return format_localized('w', 'O');
-    write1(tm_wday());
-  }
-  void on_dec1_weekday(numeric_system ns) {
-    if (ns != numeric_system::standard) return format_localized('u', 'O');
-    auto wday = tm_wday();
-    write1(wday == 0 ? days_per_week : wday);
+  void write_sign() {
+    if (negative) {
+      *out++ = '-';
+      negative = false;
+    }
   }
 
-  void on_abbr_month() {
-    if (is_classic_)
-      out_ = write(out_, tm_mon_short_name(tm_mon()));
-    else
-      format_localized('b');
-  }
-  void on_full_month() {
-    if (is_classic_)
-      out_ = write(out_, tm_mon_full_name(tm_mon()));
-    else
-      format_localized('B');
+  void write(Rep value, int width) {
+    write_sign();
+    if (isnan(value)) return write_nan();
+    uint32_or_64_or_128_t<int> n =
+        to_unsigned(to_nonnegative_int(value, max_value<int>()));
+    int num_digits = detail::count_digits(n);
+    if (width > num_digits) out = std::fill_n(out, width - num_digits, '0');
+    out = format_decimal<char_type>(out, n, num_digits).end;
   }
 
-  void on_datetime(numeric_system ns) {
-    if (is_classic_) {
-      on_abbr_weekday();
-      *out_++ = ' ';
-      on_abbr_month();
-      *out_++ = ' ';
-      on_day_of_month_space(numeric_system::standard);
-      *out_++ = ' ';
-      on_iso_time();
-      *out_++ = ' ';
-      on_year(numeric_system::standard);
-    } else {
-      format_localized('c', ns == numeric_system::standard ? '\0' : 'E');
-    }
+  void write_nan() { std::copy_n("nan", 3, out); }
+  void write_pinf() { std::copy_n("inf", 3, out); }
+  void write_ninf() { std::copy_n("-inf", 4, out); }
+
+  void format_localized(const tm& time, char format, char modifier = 0) {
+    if (isnan(val)) return write_nan();
+    const auto& loc = localized ? context.locale().template get<std::locale>()
+                                : std::locale::classic();
+    out = detail::write<char_type>(out, time, loc, format, modifier);
   }
-  void on_loc_date(numeric_system ns) {
-    if (is_classic_)
-      on_us_date();
-    else
-      format_localized('x', ns == numeric_system::standard ? '\0' : 'E');
+
+  void on_text(const char_type* begin, const char_type* end) {
+    std::copy(begin, end, out);
   }
-  void on_loc_time(numeric_system ns) {
-    if (is_classic_)
-      on_iso_time();
-    else
-      format_localized('X', ns == numeric_system::standard ? '\0' : 'E');
+
+  // These are not implemented because durations don't have date information.
+  void on_abbr_weekday() {}
+  void on_full_weekday() {}
+  void on_dec0_weekday(numeric_system) {}
+  void on_dec1_weekday(numeric_system) {}
+  void on_abbr_month() {}
+  void on_full_month() {}
+  void on_datetime(numeric_system) {}
+  void on_loc_date(numeric_system) {}
+  void on_loc_time(numeric_system) {}
+  void on_us_date() {}
+  void on_iso_date() {}
+  void on_utc_offset() {}
+  void on_tz_name() {}
+  void on_year(numeric_system) {}
+  void on_short_year(numeric_system) {}
+  void on_offset_year() {}
+  void on_century(numeric_system) {}
+  void on_iso_week_based_year() {}
+  void on_iso_week_based_short_year() {}
+  void on_dec_month(numeric_system) {}
+  void on_dec0_week_of_year(numeric_system) {}
+  void on_dec1_week_of_year(numeric_system) {}
+  void on_iso_week_of_year(numeric_system) {}
+  void on_day_of_year() {}
+  void on_day_of_month(numeric_system) {}
+  void on_day_of_month_space(numeric_system) {}
+
+  void on_24_hour(numeric_system ns) {
+    if (handle_nan_inf()) return;
+
+    if (ns == numeric_system::standard) return write(hour(), 2);
+    auto time = tm();
+    time.tm_hour = to_nonnegative_int(hour(), 24);
+    format_localized(time, 'H', 'O');
   }
-  void on_us_date() {
-    char buf[8];
-    write_digit2_separated(buf, to_unsigned(tm_mon() + 1),
-                           to_unsigned(tm_mday()),
-                           to_unsigned(split_year_lower(tm_year())), '/');
-    out_ = copy_str<Char>(std::begin(buf), std::end(buf), out_);
+
+  void on_12_hour(numeric_system ns) {
+    if (handle_nan_inf()) return;
+
+    if (ns == numeric_system::standard) return write(hour12(), 2);
+    auto time = tm();
+    time.tm_hour = to_nonnegative_int(hour12(), 12);
+    format_localized(time, 'I', 'O');
   }
-  void on_iso_date() {
-    auto year = tm_year();
-    char buf[10];
-    size_t offset = 0;
-    if (year >= 0 && year < 10000) {
-      copy2(buf, digits2(to_unsigned(year / 100)));
-    } else {
-      offset = 4;
-      write_year_extended(year);
-      year = 0;
-    }
-    write_digit2_separated(buf + 2, static_cast<unsigned>(year % 100),
-                           to_unsigned(tm_mon() + 1), to_unsigned(tm_mday()),
-                           '-');
-    out_ = copy_str<Char>(std::begin(buf) + offset, std::end(buf), out_);
+
+  void on_minute(numeric_system ns) {
+    if (handle_nan_inf()) return;
+
+    if (ns == numeric_system::standard) return write(minute(), 2);
+    auto time = tm();
+    time.tm_min = to_nonnegative_int(minute(), 60);
+    format_localized(time, 'M', 'O');
   }
 
-  void on_utc_offset() { format_localized('z'); }
-  void on_tz_name() { format_localized('Z'); }
+  void on_second(numeric_system ns) {
+    if (handle_nan_inf()) return;
 
-  void on_year(numeric_system ns) {
-    if (ns != numeric_system::standard) return format_localized('Y', 'E');
-    write_year(tm_year());
+    if (ns == numeric_system::standard) {
+      write(second(), 2);
+#if FMT_SAFE_DURATION_CAST
+      // convert rep->Rep
+      using duration_rep = std::chrono::duration<rep, Period>;
+      using duration_Rep = std::chrono::duration<Rep, Period>;
+      auto tmpval = fmt_safe_duration_cast<duration_Rep>(duration_rep{val});
+#else
+      auto tmpval = std::chrono::duration<Rep, Period>(val);
+#endif
+      auto ms = get_milliseconds(tmpval);
+      if (ms != std::chrono::milliseconds(0)) {
+        *out++ = '.';
+        write(ms.count(), 3);
+      }
+      return;
+    }
+    auto time = tm();
+    time.tm_sec = to_nonnegative_int(second(), 60);
+    format_localized(time, 'S', 'O');
   }
-  void on_short_year(numeric_system ns) {
-    if (ns != numeric_system::standard) return format_localized('y', 'O');
-    write2(split_year_lower(tm_year()));
+
+  void on_12_hour_time() {
+    if (handle_nan_inf()) return;
+    format_localized(time(), 'r');
   }
-  void on_offset_year() { format_localized('y', 'E'); }
 
-  void on_century(numeric_system ns) {
-    if (ns != numeric_system::standard) return format_localized('C', 'E');
-    auto year = tm_year();
-    auto upper = year / 100;
-    if (year >= -99 && year < 0) {
-      // Zero upper on negative year.
-      *out_++ = '-';
-      *out_++ = '0';
-    } else if (upper >= 0 && upper < 100) {
-      write2(static_cast<int>(upper));
-    } else {
-      out_ = write<Char>(out_, upper);
+  void on_24_hour_time() {
+    if (handle_nan_inf()) {
+      *out++ = ':';
+      handle_nan_inf();
+      return;
     }
-  }
 
-  void on_dec_month(numeric_system ns) {
-    if (ns != numeric_system::standard) return format_localized('m', 'O');
-    write2(tm_mon() + 1);
+    write(hour(), 2);
+    *out++ = ':';
+    write(minute(), 2);
   }
 
-  void on_dec0_week_of_year(numeric_system ns) {
-    if (ns != numeric_system::standard) return format_localized('U', 'O');
-    write2((tm_yday() + days_per_week - tm_wday()) / days_per_week);
-  }
-  void on_dec1_week_of_year(numeric_system ns) {
-    if (ns != numeric_system::standard) return format_localized('W', 'O');
-    auto wday = tm_wday();
-    write2((tm_yday() + days_per_week -
-            (wday == 0 ? (days_per_week - 1) : (wday - 1))) /
-           days_per_week);
-  }
-  void on_iso_week_of_year(numeric_system ns) {
-    if (ns != numeric_system::standard) return format_localized('V', 'O');
-    write2(tm_iso_week_of_year());
+  void on_iso_time() {
+    on_24_hour_time();
+    *out++ = ':';
+    if (handle_nan_inf()) return;
+    write(second(), 2);
   }
 
-  void on_iso_week_based_year() { write_year(tm_iso_week_year()); }
-  void on_iso_week_based_short_year() {
-    write2(split_year_lower(tm_iso_week_year()));
+  void on_am_pm() {
+    if (handle_nan_inf()) return;
+    format_localized(time(), 'p');
   }
 
-  void on_day_of_year() {
-    auto yday = tm_yday() + 1;
-    write1(yday / 100);
-    write2(yday % 100);
-  }
-  void on_day_of_month(numeric_system ns) {
-    if (ns != numeric_system::standard) return format_localized('d', 'O');
-    write2(tm_mday());
-  }
-  void on_day_of_month_space(numeric_system ns) {
-    if (ns != numeric_system::standard) return format_localized('e', 'O');
-    auto mday = to_unsigned(tm_mday()) % 100;
-    const char* d2 = digits2(mday);
-    *out_++ = mday < 10 ? ' ' : d2[0];
-    *out_++ = d2[1];
+  void on_duration_value() {
+    if (handle_nan_inf()) return;
+    write_sign();
+    out = format_duration_value<char_type>(out, val, precision);
   }
 
-  void on_24_hour(numeric_system ns) {
-    if (ns != numeric_system::standard) return format_localized('H', 'O');
-    write2(tm_hour());
-  }
-  void on_12_hour(numeric_system ns) {
-    if (ns != numeric_system::standard) return format_localized('I', 'O');
-    write2(tm_hour12());
+  void on_duration_unit() {
+    out = format_duration_unit<char_type, Period>(out);
   }
-  void on_minute(numeric_system ns) {
-    if (ns != numeric_system::standard) return format_localized('M', 'O');
-    write2(tm_min());
+};
+
+FMT_END_DETAIL_NAMESPACE
+
+#if defined(__cpp_lib_chrono) && __cpp_lib_chrono >= 201907
+using weekday = std::chrono::weekday;
+#else
+// A fallback version of weekday.
+class weekday {
+ private:
+  unsigned char value;
+
+ public:
+  weekday() = default;
+  explicit constexpr weekday(unsigned wd) noexcept
+      : value(static_cast<unsigned char>(wd != 7 ? wd : 0)) {}
+  constexpr unsigned c_encoding() const noexcept { return value; }
+};
+
+class year_month_day {};
+#endif
+
+// A rudimentary weekday formatter.
+template <typename Char> struct formatter<weekday, Char> {
+ private:
+  bool localized = false;
+
+ public:
+  FMT_CONSTEXPR auto parse(basic_format_parse_context<Char>& ctx)
+      -> decltype(ctx.begin()) {
+    auto begin = ctx.begin(), end = ctx.end();
+    if (begin != end && *begin == 'L') {
+      ++begin;
+      localized = true;
+    }
+    return begin;
   }
-  void on_second(numeric_system ns) {
-    if (ns != numeric_system::standard) return format_localized('S', 'O');
-    write2(tm_sec());
+
+  template <typename FormatContext>
+  auto format(weekday wd, FormatContext& ctx) const -> decltype(ctx.out()) {
+    auto time = std::tm();
+    time.tm_wday = static_cast<int>(wd.c_encoding());
+    const auto& loc = localized ? ctx.locale().template get<std::locale>()
+                                : std::locale::classic();
+    return detail::write<Char>(ctx.out(), time, loc, 'a');
   }
+};
 
-  void on_12_hour_time() {
-    if (is_classic_) {
-      char buf[8];
-      write_digit2_separated(buf, to_unsigned(tm_hour12()),
-                             to_unsigned(tm_min()), to_unsigned(tm_sec()), ':');
-      out_ = copy_str<Char>(std::begin(buf), std::end(buf), out_);
-      *out_++ = ' ';
-      on_am_pm();
-    } else {
-      format_localized('r');
+template <typename Rep, typename Period, typename Char>
+struct formatter<std::chrono::duration<Rep, Period>, Char> {
+ private:
+  basic_format_specs<Char> specs;
+  int precision = -1;
+  using arg_ref_type = detail::arg_ref<Char>;
+  arg_ref_type width_ref;
+  arg_ref_type precision_ref;
+  bool localized = false;
+  basic_string_view<Char> format_str;
+  using duration = std::chrono::duration<Rep, Period>;
+
+  struct spec_handler {
+    formatter& f;
+    basic_format_parse_context<Char>& context;
+    basic_string_view<Char> format_str;
+
+    template <typename Id> FMT_CONSTEXPR arg_ref_type make_arg_ref(Id arg_id) {
+      context.check_arg_id(arg_id);
+      return arg_ref_type(arg_id);
     }
+
+    FMT_CONSTEXPR arg_ref_type make_arg_ref(basic_string_view<Char> arg_id) {
+      context.check_arg_id(arg_id);
+      return arg_ref_type(arg_id);
+    }
+
+    FMT_CONSTEXPR arg_ref_type make_arg_ref(detail::auto_id) {
+      return arg_ref_type(context.next_arg_id());
+    }
+
+    void on_error(const char* msg) { FMT_THROW(format_error(msg)); }
+    FMT_CONSTEXPR void on_fill(basic_string_view<Char> fill) {
+      f.specs.fill = fill;
+    }
+    FMT_CONSTEXPR void on_align(align_t align) { f.specs.align = align; }
+    FMT_CONSTEXPR void on_width(int width) { f.specs.width = width; }
+    FMT_CONSTEXPR void on_precision(int _precision) {
+      f.precision = _precision;
+    }
+    FMT_CONSTEXPR void end_precision() {}
+
+    template <typename Id> FMT_CONSTEXPR void on_dynamic_width(Id arg_id) {
+      f.width_ref = make_arg_ref(arg_id);
+    }
+
+    template <typename Id> FMT_CONSTEXPR void on_dynamic_precision(Id arg_id) {
+      f.precision_ref = make_arg_ref(arg_id);
+    }
+  };
+
+  using iterator = typename basic_format_parse_context<Char>::iterator;
+  struct parse_range {
+    iterator begin;
+    iterator end;
+  };
+
+  FMT_CONSTEXPR parse_range do_parse(basic_format_parse_context<Char>& ctx) {
+    auto begin = ctx.begin(), end = ctx.end();
+    if (begin == end || *begin == '}') return {begin, begin};
+    spec_handler handler{*this, ctx, format_str};
+    begin = detail::parse_align(begin, end, handler);
+    if (begin == end) return {begin, begin};
+    begin = detail::parse_width(begin, end, handler);
+    if (begin == end) return {begin, begin};
+    if (*begin == '.') {
+      if (std::is_floating_point<Rep>::value)
+        begin = detail::parse_precision(begin, end, handler);
+      else
+        handler.on_error("precision not allowed for this argument type");
+    }
+    if (begin != end && *begin == 'L') {
+      ++begin;
+      localized = true;
+    }
+    end = detail::parse_chrono_format(begin, end,
+                                      detail::chrono_format_checker());
+    return {begin, end};
   }
-  void on_24_hour_time() {
-    write2(tm_hour());
-    *out_++ = ':';
-    write2(tm_min());
-  }
-  void on_iso_time() {
-    char buf[8];
-    write_digit2_separated(buf, to_unsigned(tm_hour()), to_unsigned(tm_min()),
-                           to_unsigned(tm_sec()), ':');
-    out_ = copy_str<Char>(std::begin(buf), std::end(buf), out_);
+
+ public:
+  FMT_CONSTEXPR auto parse(basic_format_parse_context<Char>& ctx)
+      -> decltype(ctx.begin()) {
+    auto range = do_parse(ctx);
+    format_str = basic_string_view<Char>(
+        &*range.begin, detail::to_unsigned(range.end - range.begin));
+    return range.end;
   }
 
-  void on_am_pm() {
-    if (is_classic_) {
-      *out_++ = tm_hour() < 12 ? 'A' : 'P';
-      *out_++ = 'M';
+  template <typename FormatContext>
+  auto format(const duration& d, FormatContext& ctx) const
+      -> decltype(ctx.out()) {
+    auto specs_copy = specs;
+    auto precision_copy = precision;
+    auto begin = format_str.begin(), end = format_str.end();
+    // As a possible future optimization, we could avoid extra copying if width
+    // is not specified.
+    basic_memory_buffer<Char> buf;
+    auto out = std::back_inserter(buf);
+    detail::handle_dynamic_spec<detail::width_checker>(specs_copy.width,
+                                                       width_ref, ctx);
+    detail::handle_dynamic_spec<detail::precision_checker>(precision_copy,
+                                                           precision_ref, ctx);
+    if (begin == end || *begin == '}') {
+      out = detail::format_duration_value<Char>(out, d.count(), precision_copy);
+      detail::format_duration_unit<Char, Period>(out);
     } else {
-      format_localized('p');
+      detail::chrono_formatter<FormatContext, decltype(out), Rep, Period> f(
+          ctx, out, d);
+      f.precision = precision_copy;
+      f.localized = localized;
+      detail::parse_chrono_format(begin, end, f);
     }
+    return detail::write(
+        ctx.out(), basic_string_view<Char>(buf.data(), buf.size()), specs_copy);
   }
-
-  // These apply to chrono durations but not tm.
-  void on_duration_value() {}
-  void on_duration_unit() {}
 };
 
-FMT_END_DETAIL_NAMESPACE
-
 template <typename Char, typename Duration>
 struct formatter<std::chrono::time_point<std::chrono::system_clock, Duration>,
                  Char> : formatter<std::tm, Char> {