Will the time always be in hours e.g. 0700, 1500 etc or will the time also have minutes e.g. 0730, 1545 etc.?
If 0700 and 1500 etc. then use DateDiff("h", start_time, finish_time) this will give you the number of hours. If the time also includes minutes you will need to use DateDiff("n", start_time, finish_time)/60 this will give a result with decimal values e.g. 133.34 hrs etc.