Simple subtracting the two numbers, as in =b1-a1 *does* work. It's your
format which is causing the problems.

You need to:
1. Use a format which includes the date as well as the time, or
2. Use a format of [hh]:mm which will display more than 24 hours.

Regards,
Fred.

"Nathan" wrote in message
...
Hi all,

I have two columns: one is the "date time" a task began and the other is
the
"date time" the task ended. I want to measure the difference in hours.

For example, in A1 I have "6/28/2008 15:44" and in B1 I have "7/1/2008
16:51" These cell values are based on a function
[=IF(AND(ISNUMBER(F66),ISNUMBER(G66)),F66+G66,"")] that has brought
together
separate date and time values from other columns.

To find the difference between these two cells, I tried simply subtracting
(which gave me another date-time value) and I tried the following formula
based on another spreadsheet I've used
[=IF(AND(ISNUMBER(A1),ISNUMBER(B1)),B1-A1,"")] but this gives me the
differences between the two times, but doesn't account for the difference
between dates.

Any thoughts?

Thanks,
Nathan