Great challenge. As others have pointed out, the wording is a bit tricky. I believe most of the problem comes from this being a formula for CHURN rate, rather than retention rate. Also the denominator would normally be All Customers Present (or Active) at the start of the month - when they actually opened their account wouldn't normally be of interest, so long as it was open before the month in question, ie the 2 year time frame is a red herring in my opinion. In this example, it doesn't affect the output though.
Sorry to ramble on but perhaps it will help others if they are confused or if they start seeing or calculating retention rates in the real world.
It was a fun challenge nonetheless and my first go at using a batch macro, so great practice for me. Thanks.
