- date: documentation ( source)
- strtotime: documentation ( source)
<?php
$hoy = strtotime(date("Y-m-d").' 04:00:00');
echo $hoy;
echo "\n";
echo strtotime(date("Y-m-d", strtotime("-1 day")).' 04:00:00');
echo "\n";
echo date('Y-m-d h:i:s');