As I said, June and November have the same number of days, so that should work fine; only months with different numbers of days have issues.
Will the dates always be the end of a month?
Ahh okay that makes sense now, I have a solution thank you! I already have a lookup built in to check whether theres 31 or 30 days in the month so will incorporate this now to ensure it calcs the right days. I misinterpreted your initial comment.