public static function makeMidnight($timestamp, $timezone=null)
	{
		$tz = new DateTimeZone($timezone !== null ? $timezone : Yii::$app->timeZone);

		$dt = new DateTime('now', $tz);
		$dt->setTimestamp($timestamp);
		$dt->setTime(0, 0, 0);

		return $dt->getTimestamp();
	}