- mktime: documentation ( source)
- date: documentation ( source)
- strtotime: documentation ( source)
<?php
echo($cd = strtotime('2013-07-16 17:30:00')."\n");
$newdate = date('Y/m/d h:i:s', mktime(date('H', $cd)+15, date('i', $cd), date('s', $cd), date('n', $cd), date('d', $cd), date('Y', $cd)));
echo($newdate."\n");