PHPC-2429: Fix UTCDateTime with negative timestamps

src/BSON/UTCDateTime.c

@@ -213,8 +213,9 @@ static PHP_METHOD(MongoDB_BSON_UTCDateTime, toDateTime)
 {
 	php_phongo_utcdatetime_t* intern;
 	php_date_obj*             datetime_obj;
-	char*                     sec;
+	char*                     sec_str;
 	size_t                    sec_len;
+	int64_t                   sec, usec;
 
 	intern = Z_UTCDATETIME_OBJ_P(getThis());
 
@@ -223,11 +224,23 @@ static PHP_METHOD(MongoDB_BSON_UTCDateTime, toDateTime)
 	object_init_ex(return_value, php_date_get_date_ce());
 	datetime_obj = Z_PHPDATE_P(return_value);
 
-	sec_len = spprintf(&sec, 0, "@%" PRId64, intern->milliseconds / 1000);
-	php_date_initialize(datetime_obj, sec, sec_len, NULL, NULL, 0);
-	efree(sec);
+	sec  = intern->milliseconds / 1000;
+	usec = (llabs(intern->milliseconds) % 1000) * 1000;
+	if (intern->milliseconds < 0 && usec != 0) {
+		/* For dates before the unix epoch, we need to subtract the microseconds from the timestamp.
+		 * Since we can't directly pass microseconds when calling php_date_initialize due to a bug in PHP,
+		 * we manually decrement the timestamp and subtract the number of microseconds from a full seconds
+		 * to store in the us field. */
+		sec--;
+		usec = 1000000 - usec;
+	}
+
+	/* TODO PHP 8.1.6+: microseconds can be included in the format string */
+	sec_len = spprintf(&sec_str, 0, "@%" PRId64, sec);
+	php_date_initialize(datetime_obj, sec_str, sec_len, NULL, NULL, 0);
+	efree(sec_str);
 
-	datetime_obj->time->us = (intern->milliseconds % 1000) * 1000;
+	datetime_obj->time->us = usec;
 }
 
 static PHP_METHOD(MongoDB_BSON_UTCDateTime, jsonSerialize)

tests/bson/bson-utcdatetime-todatetime-003.phpt

@@ -0,0 +1,33 @@
+--TEST--
+MongoDB\BSON\UTCDateTime::toDateTime() with dates before the Unix epoch
+--INI--
+date.timezone=UTC
+--FILE--
+<?php
+
+$dates = [
+    '1960-01-01 12:12:12.1',
+    '1969-12-31 23:59:59.999',
+];
+
+foreach ($dates as $date) {
+    $dateTime = new \DateTimeImmutable($date);
+    echo $dateTime->format(DateTimeInterface::RFC3339_EXTENDED), PHP_EOL;
+
+    $utcDateTime = new MongoDB\BSON\UTCDateTime($dateTime);
+
+    $newDate = $utcDateTime->toDateTime();
+    echo $newDate->format(DateTimeInterface::RFC3339_EXTENDED), PHP_EOL, PHP_EOL;
+}
+
+?>
+===DONE===
+<?php exit(0); ?>
+--EXPECT--
+1960-01-01T12:12:12.100+00:00
+1960-01-01T12:12:12.100+00:00
+
+1969-12-31T23:59:59.999+00:00
+1969-12-31T23:59:59.999+00:00
+
+===DONE===