Nanosecond-aware date/time formatters

Standard strftime() and strptime() do not handle subsecond values. Python allows "%f" to indicate microseconds; the functions in this library extend that capability to nanoseconds.

#include <assert.h>
#include <string.h>

#include "strtime.h"

int main() {
  struct tm tm_;
  int nanos;

  memset(&tm_, 0, sizeof(tm_));
  nanos = 0;

  char timestr[] = "2019-03-25 05:30:24.051421000";
  strptime_ns(timestr, "%Y-%m-%d %H:%M:%S.%f", &tm_, &nanos);

  char buffer[80];
  strftime_ns(buffer, sizeof(buffer), "%Y-%m-%d %H:%M:%S.%f", &tm_, nanos);

  assert(strcmp(timestr, buffer) == 0);
  return 0;
}

These routines are based on tzcode from Android's Bionic, available under the BSD license.

Infer datetime format from string

One difficulty with strtime functions is that they require a precise format string. A quick-and-dirty solution is provided in this library.

char format[80];
istrtime(timestr, format, sizeof(format));

strptime_ns(timestr, format, &tm_, &nanos);
strftime_ns(buffer, sizeof(buffer), format, &tm_, nanos);

Fast timegm() and gmtime()

The system timegm() handles quirks of the Gregorian calendar that we don't need. Similarly, the system gmtime() on Windows does not handle negative clocks (pre-epoch dates). This library offers faster functions that can handle a fuller range of timestamps.

time_t clock = fast_timegm(&tm_);
fast_gmtime(&clock, &tm_);

By necessity, this only works for UTC; timezones and daylight savings are too costly.