- mktime: documentation ( source)
- date: documentation ( source)
- strtotime: documentation ( source)
<?php
$days = 41397 - 25569;
$unix = strtotime("+$days days", mktime(0, 0, 0, 1, 1, 1970));
$dt = DateTime::createFromFormat('d/m/Y', date('d/m/Y', $unix));
echo $dt->format('d/m/Y');