Calculate time difference for planned v actual times

i have a spreadsheet which i want to calculate the variance between planned v actual times, i have following columns
Planned start, actual start, planned finish, actual finish, total hours, total planned hours and hours variance
the problem starts when i have a finish time after midnight, keeps returning negative total hours eg

planned start 13:00
Actual start 13:00
Planned finish 00:30
Actual finish 00:30
Total planned hours is returning -12:30, same with actual hours. I have excel using the 1904 date system but cant get this to work, anyone have any ideas?

Lawry

Time is stored as a floating-point number between 0 and 1.

So, something like thirty minutes past midnight would be something like 0.02 and 13:00 would be about 0.54
0.54 - 0.02 = -0.52 => negative 12 1/2 hours

Two solutions:
1) Add date information
The reason time is stored as a floating point between 0 and 1 is becuase dates are stored as integers from day 1 at the start of the calendar to now; that way date+time can be stored as one big ol' number in the cell.
1000.02 - 999.54 = the correct output

2) Add assumptions
=IF(start_time <= end_time, end_time - start_time, 24_hours - end_time + start_time)
This will assume that if the end_time happens "before" the start_time, it's actually the next day.