Calculating Latest Common Day of the Month

[ElPresidente]

Hello wise Excel experts! I humbly submit my problem.

I need to calculate a due date based on payments made. So, essentially I need the latest common date in several date ranges that will be based of payment dates.

Here are the rules.
1 payment per month, although there may be multiple actual payments in a month.
Each payment can have a due date + or - 20 days from the actual payment date.
The latest due date accepted is the 28th, if 20 days back is 31st - 29th the farthest back we can go is the 1st. Example a 2/18 payment can only go back to 2/1 NOT 1/29. The converse is also true if 20 days forward is 29th - 31st the payment can only go to the 28th.

If I have payments on the following dates:

4/11/2014
4/22/2014
5/16/2014
6/23/2014
7/15/2014


My common day would be the 4th starting on 3/4 going to 7/4.

Thanks in advance for helping a mindless boob like me.

[Jacc]

If you post a sample worksheet with your expected results it will be much easier to help you.