You talked about DateDiff, so I'm assuming your values are in Date variables (or DateTime fields).
First, calculate the difference and save it in a Date variable:
Dim duration As Date
duration = stoptime - starttime
If you want to round to the nearest minute, use
duration = stoptime - starttime + 3.47222222222222E-4
The constant is equivalent to 30 seconds expressed as a fraction of a day.
If the difference is limited to less than a day, the most efficient way is to simply format the difference:
something = Format(duration, "Short Time"

This gives you a string you can put into a text box control, for example.
If you want to store the result, just save the Date variable (duration). When you're ready to display it in a datasheet, form, or report, set the Format function to "Short Time".
If the difference might be a day or more, but you still want it in hours and minutes, you can't use the Format function. Instead, use
hours = Int(duration * 24#)
minutes = ((duration * 24#) - hours) * 60
result = CStr(hours) & ":" & Format(minutes, "00"

Rick Sprague