- time: documentation ( source)
- date: documentation ( source)
- strtotime: documentation ( source)
<?php
$timeStamp = strtotime(time());
$ayer = time() - 1 * 24 * 60 * 60;
$ayer = 1377921600;
echo PHP_EOL;
echo date(DATE_ATOM, $ayer);
echo strtotime('2013-07-29 04:00:00 UTC');