assuming you had two different epoch times $t1 and $t2 you could...

$diff =  $t2 - $t1;
$ss = $diff % 60;
$diff = ($diff - $ss) / 60;
$mi = $diff % 60;
$hh = ($diff - $mi) / 60;
sprintf("%d:%02d:%02d" , $hh, $mi, $ss);
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This gives hours:minutes:seconds. I'll leave it as an exercise for you to add the days calculation.