I would suggest you to look at
TIMEDIFF[
^] function.
PHP way: you need to use mktime functions:
example would be:
<?php
$time1="2014-04-15 10:10:12";
$time2="2014-04-15 12:10:12";
$time1_int=strtotime ($time1);
$time2_int=strtotime ($time2);
$diff=$time2_int-$time1_int;
$sec=$diff%60;
$minitue=floor($diff/60)%60;
$hour=floor(floor($diff/60)/60);
echo $hour.":".$minitue.":".$sec;
There might be far better solution for this.