Excel date & time frustration

I am working in Excel 97, and trying to calculate the difference between two dates and times. I have ‘time in’ in cells A1:A2 – ‘time out’ in cells B1:B2. I calculate the difference between the in and out dates in cell C1:C2 — =B1-A2 formatted as d:h:mm. This does not calculate the days correctly unless the dates are in the same month (see below) Also, when I try to sum the differences: =sum(C1:C2) (26:1:53+22:22:32), I get the crazy answer of 20:0:25. Whatam I missing? How can get the difference between the time in and the time out in the d:h:mm format to display correctly, and then how can I sum those differences??

Time In
09/19/01 9:21 AM
09/19/01 11:40 AM

Time Out
11/15/01 11:14 AM
11/12/01 10:12 AM

Difference (format d:h:mm)
26:1:53
22:22:32