Excel EDATE
The first method we'll use requires the EDATE function/formula, which is a standard function in Excel 2007.
In previous versions of Excel, EDATE is a part of the Excel Analysis Toolpak. To make sure the Analysis Toolpak is installed, select Tools → Add-ins and check Analysis Toolpak).
Here's howto can use the EDATE function to add one month to the date in A1 (31-Aug-2007).
First, click in cell A1 and type in the date 31-Aug-2007. Then, click in B1 and enter in the following formula:
=EDATE(A1,1)
This formula yields a result of 30-Sep-2007, rather than 31-Sep-2007, because there are only 30 days in September.
To subtract month from the same date, you can use this formula:
=EDATE(A1,-1)
This yields the result 31-Jul-2007. This means that by using EDATE you are adding or subtracting a calendar month to your date. This may or may not be what you'd expect, depending on how many days you define a month as having.
