Time

I need to compare actual time to a deadline and calculate the minutes early or late - and across midnight. For example; if the deadline is 11:50 pm and the actual is 11:55 pm the result is 5 minutes late, if the actual is 11:45 pm it's 5 minutes early. Where I'm having trouble is when the deadline is 11:50 pm and the actual time is 12:05 am (15 minutes late). I've tried several formulas (see below) but none will work uniformly across all three situations. Can someone offer a suggestion?

Some formulas that work some of the time (but not all):

=B1+(A1>B1)-A1
=B1-A1+(b1<A1)
=B1-A1+IF(A1>B1,1)

?? Thanks...

If there's not more than a day's difference ever, you can use =Mod(B1-A1,1)