Excel Formula to find a date that is within 20 days of a series of dates.

Need to be able to find what day of the month would be within 20 days of a series of monthly payments.

Say you make monthly payments willy nilly with no regard to due date and then after 9 months you want to set a due date that will still show your old payments were on time. That is what i am trying to do.

I've attached the workbook. Sheet2 is the calculator we use, sheet 1 has a little more explanation and demonstration.

Typically Column "C" is hidden. But I didn't want anyone to miss it.

The far left column has the months, but that would be less important if I could find the common date. Because a January 30th payment may count for February if the due dates or the 2nd of every month.