Add/Subtract n Months from a Date One common request is to use Excel to add any desired number of months to a date. For example, if A1 houses the date 31-Aug-2005 you may wish to add (or take) 1 month to that date. There is always going to be controversy as to what number of days should be added/taken to represent a month. This is because not all months have the same number of days.