I can calculate the expiration date manually, but I need a formula. I use the information constantly but don't know how to set it up in excel.

John Doe's parole expires on 2/1/2008.
Each month on the 9th the parole date is reduced by 6 days.
If his parole expires before the 9th of the month, he will get 6 days on the 20th of the previous month.
When will John Doe's parole expire?
***the 6 represents a variable. It could be a 10, 12, 16 , 6****
The correct answer, calculated manually using the 6 is 12-27-2007.

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The above does not work if I extend the start date to 2/1/2010 and change the compensation to 16.
The correct answer should be 9-26-2009.