- abs: documentation ( source)
- date: documentation ( source)
- strtotime: documentation ( source)
<?php
$startTimeStamp = strtotime("1971/01/01");
$endTimeStamp = date("d/m/Y");
$timeDiff = abs($endTimeStamp - $startTimeStamp);
$numberDays = $timeDiff/86400; // 86400 seconds in one day
// and you might want to convert to integer
$numberDays = intval($numberDays);
?>