Add Months (EDATE)
This is an alternative to EDATE if you don't have the Analysis Toolpak installed.
=IF(DAY(A1)<>DAY(DATE(YEAR(A1),MONTH(A1)+B1,DAY(A1))),DATE(YEAR(A1),MONTH(A1)+B1+1,0),DATE(YEAR(A1),MONTH(A1)+B1,DAY(A1)))
B1 = Months to add
This formula avoids problems with different numbers of days in a month. It will either return the same day in the month added by B1, or it will calculate to the end of that month. (No overlapping into the month immediately after)
I also saw this formula posted on a forum. It works in a simliar way to my formula above.
=DATE(YEAR(A1),MONTH(A1)+B1,MIN(DAY(A1),DAY(DATE(YEAR(A1),MONTH(A1)+B1+1,0))))