I need to calculate an expiration date.

I want to calculate the date of expiration from the current date, whatever it might be.
I want to be able to enter one of the folowing variables, whicever may apply at the time: 4,6,10,12,14,16.
Simple Example:
today is 2/22/2009
expiration date is 5-1-2009
variable is 16
the date will be reduced by the variable on the 1st day of each month unless the date computed will be less than the 1st, then it will be calculated on the 20th of the previus month.


on 3-1-2009 5-1-2009(-16) = 4-15-2009
on 3-20-2009 4-15-2009(-16) = 3-30-2009
The variable above, of course in this instance is 16.

correct answer is 3-30-2009

It is easy to calculate by hand unless the expiration date is 1 or more years in the future.

Complicated Example: expiration 5-20-2019
Variable 12
Then I may need to calculate based on a 16 variable, etc.