Enforce UTC timestamps, fixes #3951

Author: MGatner

system/I18n/Time.php

@@ -297,7 +297,7 @@ public static function createFromFormat($format, $datetime, $timeZone = null)
 	 */
 	public static function createFromTimestamp(int $timestamp, $timezone = null, string $locale = null)
 	{
-		return new Time(date('Y-m-d H:i:s', $timestamp), $timezone, $locale);
+		return new Time(gmdate('Y-m-d H:i:s', $timestamp), $timezone ?? 'UTC', $locale);
 	}
 
 	//--------------------------------------------------------------------

tests/system/I18n/TimeTest.php

@@ -212,7 +212,15 @@ public function testCreateFromFormatWithInvalidFormat()
 
 	public function testCreateFromTimestamp()
 	{
-		$time = Time::createFromTimestamp(strtotime('2017-03-18 midnight'));
+		// Se the timezone temporarily to UTC to make sure the test timestamp is correct
+		$tz = date_default_timezone_get();
+		date_default_timezone_set('UTC');
+
+		$timestamp = strtotime('2017-03-18 midnight');
+
+		date_default_timezone_set($tz);
+
+		$time = Time::createFromTimestamp($timestamp);
 
 		$this->assertEquals(date('2017-03-18 00:00:00'), $time->toDateTimeString());
 	}