Converting Elapsed Time to Minutes

I want to compute elapsed time (currently showing as H:M) to minutes. A1=StartTime, B1=EndTime, and C1=ElapsedTime. The formula in C1 is =HOUR(B1-A1)*60+MINUTE(B1-A1), and it works fine as long as I don't have a PM time in A and an AM time in B (or vice versa); otherwise I am getting an overflow error (Excel seems to have a problem with what it sees as a negative result). I have tried to remedy this by using the military time standard (24 hour basis) but I still get the error. Does anyone have a solution to this problem? Many thanks.

http://www.cpearson.com/excel/datearith.htm

Look at the section entitled "Subtracting Times". It explains WHY this is happening and a solution.