You really did upgrade (pre Excel 95). Excel does not support negative times.
If it is a matter of getting a difference in time through midnight
=C2 - B2 + (B2 > C2)
the (B2>C2) is a logical expression returns 0 or 1
since time is a fraction of a day the 1 would be adding 24 hours.
If you want functions that deal with dates before 1900 and that
deal with negative time you would need to use VBA, and John Walkenbach
has a formulas for that Extended Date and Time Functions
http://www.j-walk.com/ss/excel (use the search)