What kind of times are we speaking of? In what format do you want your results, i.e. minutes... hours and minutes... days, hours and minutes?
Generally speaking, you use DateDiff(), as has been suggested, using the smallest unit of time you want to report. If you want hours and minutes you use minutes as the Interval in DateDiff() and then parse it out into hours and minutes.
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The problem with making anything foolproof...is that fools are so darn ingenious!
All posts/responses based on Access 2003/2007