Okay; so the exact date doesn't matter, and nor for that matter does the exact time (we just want durations).
Start a running total: $total=0;
Go through each $duration in turn - you're getting them in HH:MM:SS format? A quick way to break that into $hours, $minutes, $seconds is
list($hours,$minutes,$seconds)=explode(':',$duration);
Now we convert the whole duration to seconds:
$seconds=$seconds+60$minutes+3600$hours;
And add that to the running total:
$total+=$seconds;
At the end of the loop, you've got a total of seconds used in the month. Break that back out into days, hours, minutes, seconds. I was going to do this by hand with a lot of / and % operations, but since we're not going to be spanning over a whole year, let's just pretend that it's a real timestamp from January, 1970 (hey, 5 days and 6 hours is 5 days and 6 hours) and use date():
$totalduration=date('z:H:i:s',$total);
And if you want those in separate fields:
list($days,$hours,$minutes,$seconds) = explode(':',$totalduration);
Howzat?
