Account for different handling of historical dates by ICU and Posix

src/php-8.1-strftime.php

@@ -6,6 +6,7 @@
   use DateTimeInterface;
   use Exception;
   use IntlDateFormatter;
+  use IntlGregorianCalendar;
   use InvalidArgumentException;
 
   /**
@@ -85,7 +86,18 @@ function strftime (string $format, $timestamp = null, ?string $locale = null) :
           $pattern = $intl_formats[$format];
       }
 
-      return (new IntlDateFormatter($locale, $date_type, $time_type, $tz, null, $pattern))->format($timestamp);
+      // In October 1582, the Gregorian calendar replaced the Julian in much of Europe, and
+      //  the 4th October was followed by the 15th October.
+      // ICU (including IntlDateFormattter) interprets and formats dates based on this cutover.
+      // Posix (including strftime) and timelib (including DateTimeImmutable) instead use
+      //  a "proleptic Gregorian calendar" - they pretend the Gregorian calendar has existed forever.
+      // This leads to the same instants in time, as expressed in Unix time, having different representations
+      //  in formatted strings.
+      // To adjust for this, a custom calendar can be supplied with a cutover date arbitrarily far in the past.
+      $calendar = IntlGregorianCalendar::createInstance();
+      $calendar->setGregorianChange(PHP_INT_MIN);
+
+      return (new IntlDateFormatter($locale, $date_type, $time_type, $tz, $calendar, $pattern))->format($timestamp);
     };
 
     // Same order as https://www.php.net/manual/en/function.strftime.php

tests/strftimeTest.php

@@ -176,4 +176,30 @@ public function testLocale () {
       $result = strftime('%B', '20220306 13:02:03', 'eu');
       $this->assertEquals('martxoa', $result, '%B: Full month name, based on the locale');
     }
+
+    /**
+     * In October 1582, the Gregorian calendar replaced the Julian in much of Europe, and
+     *   the 4th October was followed by the 15th October.
+     *  ICU (including IntlDateFormattter) interprets and formats dates based on this cutover.
+     *  Posix (including strftime) and timelib (including DateTimeImmutable) instead use
+     *   a "proleptic Gregorian calendar" - they pretend the Gregorian calendar has existed forever.
+     *  This leads to the same instants in time, as expressed in Unix time, having different representations
+     *   in formatted strings.
+     */
+    public function testJulianCutover () {
+      // 1st October 1582 in proleptic Gregorian is the same date as 21st September 1582 Julian
+      $prolepticTimestamp = DateTimeImmutable::createFromFormat('Y-m-d|', '1582-10-01')->getTimestamp();
+      $result = strftime('%x', $prolepticTimestamp, 'eu');
+      $this->assertEquals('82/10/1', $result);
+
+      // In much of Europe, the 10th October 1582 never existed
+      $prolepticTimestamp = DateTimeImmutable::createFromFormat('Y-m-d|', '1582-10-10')->getTimestamp();
+      $result = strftime('%x', $prolepticTimestamp, 'eu');
+      $this->assertEquals('82/10/10', $result);
+
+      // The 15th October was the first day after the cutover, after which both systems agree
+      $prolepticTimestamp = DateTimeImmutable::createFromFormat('Y-m-d|', '1582-10-15')->getTimestamp();
+      $result = strftime('%x', $prolepticTimestamp, 'eu');
+      $this->assertEquals('82/10/15', $result);
+    }
   }