If you need to calculate a date in Excel that is a certain number of days in the future, you can simply enter a formula adding the correct number of days to a date.  For example, if you wanted to calculate a due date for an invoice with terms of net 30, and the invoice date was in cell B2, you could use the formula =B2+30. That will give you a date 30 days from the date in cell B2.

What if you want to get the same date one or more months in the future, such as a next billing date?  In this case you can use a function named EDATE.  The format is simple.  It’s just EDATE(start_date,months).  For example, if the last invoice date (1/15/2012) is in cell B2 and you need to find the next quarterly billing date, you could use the formula =EDATE(B2,3).  That will give you 4/15/2012, the date 3 months from 1/15/2012.  If the date in B2 was 1/31/2012, the formula’s result would be 4/30/2012, since there are only 30 days in April.

With either approach you may get a number instead of something that looks like a date.  This is because Excel tracks dates as a serial number with January 1st, 1900 as day 1.  If that happens, right click on the cell, choose Format Cells, and select a date format from the list.

To calculate a prior date, subtract the number of days, or use a negative number for the months if using EDATE.