Fix GH-21616: DateTime::modify() wrong timestamp across DST transitions

ext/date/php_date.c

@@ -3288,6 +3288,7 @@ static bool php_date_modify(zval *object, char *modify, size_t modify_len) /* {{
 	php_date_obj *dateobj;
 	timelib_time *tmp_time;
 	timelib_error_container *err = NULL;
+	timelib_sll rel_h, rel_i, rel_s, rel_us;
 
 	dateobj = Z_PHPDATE_P(object);
 
@@ -3356,8 +3357,36 @@ static bool php_date_modify(zval *object, char *modify, size_t modify_len) /* {{
 
 	timelib_time_dtor(tmp_time);
 
+	/* do_adjust_relative() applies h/i/s as wall-clock, which breaks across
+	 * DST. Strip them before timelib_update_ts and re-apply via SSE below. */
+	rel_h  = dateobj->time->relative.h;
+	rel_i  = dateobj->time->relative.i;
+	rel_s  = dateobj->time->relative.s;
+	rel_us = dateobj->time->relative.us;
+	dateobj->time->relative.h  = 0;
+	dateobj->time->relative.i  = 0;
+	dateobj->time->relative.s  = 0;
+	dateobj->time->relative.us = 0;
+
 	timelib_update_ts(dateobj->time, NULL);
 	timelib_update_from_sse(dateobj->time);
+
+	/* Normalize microseconds: fold full seconds into rel_s, keep rel_us >= 0 */
+	rel_s  += rel_us / 1000000;
+	rel_us  = rel_us % 1000000;
+	if (rel_us < 0) {
+		rel_s--;
+		rel_us += 1000000;
+	}
+
+	dateobj->time->sse += timelib_hms_to_seconds(rel_h, rel_i, rel_s);
+	dateobj->time->us  += rel_us;
+	if (dateobj->time->us >= 1000000) {
+		dateobj->time->us -= 1000000;
+		dateobj->time->sse++;
+	}
+	timelib_update_from_sse(dateobj->time);
+
 	dateobj->time->have_relative = 0;
 	memset(&dateobj->time->relative, 0, sizeof(dateobj->time->relative));
 

ext/date/tests/date_modify-1.phpt

@@ -25,4 +25,4 @@ Sun, 22 Aug 1993 00:00:00 +12
 Sun, 27 Mar 2005 01:59:59 CET
 Sun, 27 Mar 2005 03:00:00 CEST
 Sun, 30 Oct 2005 01:59:59 CEST
-Sun, 30 Oct 2005 03:00:00 CET
+Sun, 30 Oct 2005 02:00:00 CET

ext/date/tests/gh15880.phpt

@@ -0,0 +1,28 @@
+--TEST--
+Bug GH-15880 (DateTime::modify('+72 hours') incorrect across DST boundary)
+--FILE--
+<?php
+/* Fall back: America/Chicago, 2024-11-03 02:00 CDT -> 01:00 CST.
+ * +72 hours from midnight Nov 1 must land at Nov 3 23:00 CST, not Nov 4 00:00. */
+date_default_timezone_set('America/Chicago');
+$tz = new DateTimeZone('America/Chicago');
+$start = new DateTimeImmutable('2024-11-01 00:00:00', $tz);
+
+/* modify and add must agree */
+echo $start->modify('+72 hours')->format('Y-m-d H:i:s T U'), "\n";
+echo $start->add(new DateInterval('PT72H'))->format('Y-m-d H:i:s T U'), "\n";
+
+/* +3 days is calendar arithmetic -- it should land on midnight Nov 4 */
+echo $start->modify('+3 days')->format('Y-m-d H:i:s T U'), "\n";
+
+/* -72 hours backward through fall-back: 73 real hours separate Nov 1 00:00 CDT
+ * from Nov 4 00:00 CST (the extra hour is the repeated hour), so -72h lands 1h
+ * ahead of Nov 1 midnight */
+$end = new DateTimeImmutable('2024-11-04 00:00:00', $tz);
+echo $end->modify('-72 hours')->format('Y-m-d H:i:s T U'), "\n";
+?>
+--EXPECT--
+2024-11-03 23:00:00 CST 1730696400
+2024-11-03 23:00:00 CST 1730696400
+2024-11-04 00:00:00 CST 1730700000
+2024-11-01 01:00:00 CDT 1730440800

ext/date/tests/gh21616.phpt

@@ -0,0 +1,42 @@
+--TEST--
+Bug GH-21616 (DateTime::modify() does not respect DST transitions)
+--FILE--
+<?php
+/* Spring forward: Europe/London, 2025-03-30 01:00 GMT -> 02:00 BST */
+$tz = new DateTimeZone('Europe/London');
+
+/* +1s then -1s must round-trip */
+$dt = new DateTime('2025-03-30 00:59:59', $tz);
+$dt->modify('+1 second');
+$dt->modify('-1 second');
+echo $dt->format('Y-m-d H:i:s T U'), "\n";
+
+/* -1s from 02:00 BST must land at 00:59:59 GMT, not 02:59:59 BST */
+$dt2 = new DateTime('2025-03-30 02:00:00', $tz);
+echo $dt2->modify('-1 second')->format('Y-m-d H:i:s T U'), "\n";
+
+/* month + hours: +1 month lands before the DST boundary, so +1 hour is plain GMT */
+$dt3 = new DateTime('2025-02-28 00:30:00', $tz);
+echo $dt3->modify('+1 month +1 hour')->format('Y-m-d H:i:s T U'), "\n";
+
+/* first/last day of must still work */
+$base = new DateTimeImmutable('2025-03-15 10:00:00', $tz);
+echo $base->modify('first day of next month')->format('Y-m-d H:i:s T'), "\n";
+echo $base->modify('last day of this month')->format('Y-m-d H:i:s T'), "\n";
+
+/* +61 minutes from just before the gap -- minutes must also count as elapsed time */
+$dt4 = new DateTime('2025-03-30 00:59:00', $tz);
+echo $dt4->modify('+61 minutes')->format('Y-m-d H:i:s T U'), "\n";
+
+/* DateTimeImmutable must behave the same as mutable DateTime */
+$dt5 = new DateTimeImmutable('2025-03-30 02:00:00', $tz);
+echo $dt5->modify('-1 second')->format('Y-m-d H:i:s T U'), "\n";
+?>
+--EXPECT--
+2025-03-30 00:59:59 GMT 1743296399
+2025-03-30 00:59:59 GMT 1743296399
+2025-03-28 01:30:00 GMT 1743125400
+2025-04-01 10:00:00 BST
+2025-03-31 10:00:00 BST
+2025-03-30 03:00:00 BST 1743300000
+2025-03-30 00:59:59 GMT 1743296399