- strtotime: documentation ( source)
<?php
$s = strtotime($star);
$f = strtotime($finish);
$e = $f-$s;
echo sec2date($e);
function sec2date($secs) {
$sec = $secs%60;
$jam = intval($secs/3600);
$menit = intval($secs/60)%60;
$sec = ($sec < 10)?"0$sec":"$sec";
$jam = ($jam < 10)?"0$jam":"$jam";
$menit = ($menit < 10) ? "0$menit" : "$menit";
return "$jam:$menit:$sec";
}