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