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