Thank Al for your reply.
I believe that I correctly did not explain myself what I want is subtraction or to add two hours for example:
12:00:00 p.m. - 9:0:00 is equal to 3 hours.
I mean the intensity is make a form to calculate the extra time the my employers work, for example:
Peter Mckarly work the saturday, June 23 from 8:00: a.m. to 12:00:00 p.m.
In the form the user has to put the Peter id the date (Saturday, June 23) and in a grid the start to (8:00:00 a.m.) Finish to (12:00:00).
I hope you can help and thanks again.