Add Date Arithmetic output field to Excel Macro

I have an existing Excel Office 2002 macro that converts columns from an input .csv file into a particular format .txt file, to load items into Google Base. This macro currently copies the expiration date from the input file, but this fixed date is too far in the future for Google base loads. I would like you to modify the macro to put in a new expiration date that is an input parameter between 1 and 30 days from the current date. To do this you have to correctly implement date arithmetic in an Excel macro. For example if today is the 30th of May, and the input date parameter is 5, the calculated date would be May 30th plus 5 days, which would be June 4th. Different month have differing numbers of days and at the end of the year, dates roll into the next year. Please do not bid if you do not know how to do date arithmetic in an Excel Macro.