Substract two times, total hours and working hours

Question

I want to substract two hours:

a. 40:00:00 b. 31:18:00

I want this result 09:42:00. How can i achieve this?

Answer 1

The result is 08:42:00 (40:00:00 - 31:18:00 = 08:42:00 not 09:42:00)

I think this solves:

$lasthour = '40:00:00';
$firsthour =  '31:18:00';

//explode first hour
list($h, $m, $s) = explode(':', $firsthour);
//convert to seconds 
$secFh = ($h * 3600 + $m * 60 + $s);

//explode last hour
list($h, $m, $s) = explode(':', $lasthour);
//convert to seconds 
$secLh = ($h * 3600 + $m * 60 + $s);

$sub = $secLh - $secFh;

echo gmdate("H:i:s", $sub);