Date and Time Calculations

Hi I am a contractor and i have employees on several jobs in an area so that means a employee can work from 6:00 AM to 9:00 AM on one job and then go to another job and work from 9:30 AM to 12:30 PM and so on. What i need to do is figure out how to make excel calculate hours on the first job and then calculate how many on the next job.
A B C
1 In Out Discription
2 6:00 AM 9:00 AM Worked on windows
3 9:30 AM 12:00 PM Installed new door knobs
4


Thanks

superclark