General :: Create Time Billing Form

What I am trying to do, is create a time billing form to track time spent on different tasks, with the following items in a neat form, in this format:

1) Client
2) Employee
3) Rate/hour
4) Date
5) StartTime
6) EndTime
7) BillableHours (*)

I am able to make a form that correctly works lets the user enter records for 1-6, with the user pressing crtl+shft+; to give the correct starttime, and then at the end of the task, the user can press the same and register the end of the task.

My problem is that I can't work out how to calculate 7) as one needs to input some VBA code that converts date/time to numeric values, allowing you to calculate time difference. However, I simply don't know; a) the proper 'code' that will calculate the time different between 5) and 6). I saw that it was perhaps the dateDiff command? And b) where I would even put that code into, i.e. the exact location where one goes to and types the code into... Is it a query? Do you set it to a Calculated value in the table editor?