My job requires a lot of calculation involving elapsed time in whole months. I use DATEDIF all the time to do this. I recently ran into one that doesn't make sense to me.

Start Date: 01/01/1987
End Date: 09/30/1987

=DATEDIF(01/01/1987,09/30/1987,"M") returns a result of 8 months. I should be getting a result of 9 though, the completion of the month of September should grant a month. I tweaked the formula a bit to use a start date of 12/31/1986 but it still returned a result of 8 months. The only way I was able to get a result of 9 months was to use a start date of 12/30/1986 or an end date of 10/01/1987. I can't really do either of those things. Why isn't this working as expected.

I didn't have any better luck trying to use YEARFRAC either. Does anyone have a better idea for elapse time functions?

Thanks a bunch.