I agree with SteveG's diagnosis of the Analysis Toolpak. However, I
wonder if you don't need a slightly different solution, and here's why:
when I mocked up your scenario and used EDATE with a start date of
1/30/2006 the result was 2/28/2006. The EDATE result for the following
months was 3/28, 4/28 etc, not producing your desired output of 3/30,
4/30.

Are you *always* trying to produce a date on the 30th of the month, or
is that just an example?