Darren -
Remember that time values are actually a special format of a date. It
is likely that unless specifically entered with the date, the default
date for a time value is 1/0/1900. If you enter 10:00 PM as the
StartTime and 1:00 AM as the EndTime, Excel will think that both are
from the same day which means the EndTime will have a smaller value
than the StartTime. That is why your formula doesn't work when the
time crosses midnight. Here's a fix.

Assuming that you time values are within 24 hours of each other the
following formula will work:

=IF(EndTime<StartTime, 1 + EndTime - StartTime, EndTime - StartTime)

This formula will add a day to the end time so instead of being 1:00 am
on 1/0/1900 it will be 1:00 am on 1/1/1900 (a day later). The result
of the formula is the portion of a 24 hour period that has transpired
between the two times. You'll still need to convert to hours or
minutes.

If you have time periods that span multiple days, you'd take a slightly
different approach. Add a column that contains EndDate. Then your
formula would be:
= (EndDate+EndTime) - StartDate+StartTime)

Hope that helps.

- John Michl