Help with a time calculation....

[fotomatt1]

I'm not sure if this is a calculation question or a script question, but I'm hoping that some of you can help. I'm creating a simple database so my employees can track how long it takes them to complete certain projects. Their work on these projects starts and stops several times, so I was hoping to create one button that basically starts & stops the clock, and each time they start and stop the clock the time for that segment gets added to the previous block of time. This way I can have one field for the total time spent on that project. I hope I'm not being confusing, but any help with this will be greatly appreciated, as I've been working on this for a couple of days and can't seem to figure it out.

[comment]

each time they start and stop the clock the time for that segment gets added to the previous block of time.

Not a good arrangement, IMHO. There is no trail left and if a mistake is made, all data will be lost. Better create separate records in another table with start and stop timestamps for every segment. The total time will be the sum of the segments' durations.