time calculation

[Mariusz]

What's the best way to find the difference in hours between $checkin_time and 
$checkout_time?
I thought each hour from 12am through 12pm would supply integer from 0 to 24 (e.g. 9am 
to 5pm is 17-9=8) but that does work except when checkout is 12am.(I don't have to 
deal with minutes in this script)
Thanks
Mariusz