In case you use a fields in Time format, only the the current time is entered and entries matches specific conditions (p.e. you have an log form, where you register event's start and end times, no event can take more time as 23:59:59, and you have on form a disabled control for time with default as NOW(), then you can calculate event duration as
Code:
Iif(EventEndTime<EventStartTime, 1, 0) + EventEndTime - EventStartTime
Of-course you can use this only for certain log type tables. When start and end times are recorded in same row, or time difference can be 24 hours or more, you have to use datetime fields as Bob adviced.