- date_add: documentation ( source)
- date_interval_create_from_date_string: documentation ( source)
<?php
$t1 = 1400742000;
$dt = DateTime::createFromFormat('U', $t1);
echo $dt->format('H:i:s'), PHP_EOL;
date_add($dt, date_interval_create_from_date_string('1 hour'));
echo $dt->format('H:i:s'), PHP_EOL;