- assert: documentation ( source)
<?php
$tz = new \DateTimeZone('Europe/Moscow');
$dt = new DateTimeImmutable('now');
$dt->setTimezone($tz);
$timestamp = (int) $dt->format('U');
$timestamp += 1000;
$dt2 = \DateTimeImmutable::createFromFormat('U', $timestamp, $tz);
assert($dt->getTimezone() === $dt2->getTimezone());