Sequential Formula for months based on a 4, 4, 5 schedule

Hi,

I am looking to put a formula (which will be ultimately nested within a larger formula for a lookup) that tracks the life of a Cohort. In the attached file, Row 1 are listed Cohorts and the month the Cohort is in each week. Since months are 4.33 weeks, I want the third month of the quarter to assume 5 weeks and all other months 4 weeks.

Cohort 02/01/2016's 52nd week would be in January 2017. Also I don't have any values past 12/1/2020, so once that time passes .. just cycle through the 2020 months again.

Thanks in advance for the help!

Apolgoies if I wasn't clear within my post. The file attached in my original post is what I am looking to do. I will be using this to do a lookup. So I need to pull in a margin value at each Cohort's X week. So, for Cohort 01/01/2016 (Column C) there 52nd week falls in December of 2016. And because there aren't an equal number of weeks in a month, I want the first two months of a quarter just to be 4 weeks and the third month of the quarter t obe 5 months.

Resurfacing for visibility

.... margin value at each Cohort's X week

I expect many of us (especially me!) have'nt clue what this means: define Cohort?

Sorry, I just meant Cohort being the start period for a group of people. That's row 1 within this file. Thats when the life of those group of people start. That's why Cohort 03/01/2016 first 5 weeks (because it's third month in the quarter) are all 3/1/2016.

Here's a similar post (https://www.excelforum.com/excel-for...-13-weeks.html) I had asked for but on a quarterly basis.