finding max date for a particular month

Happy New Year to you all

I have a sheet that has a list of dates in column B, can be multiple instances, I have the following line that findes the end date based on a previously defined date

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this looks for the last working date of the previous month, but I've had a problem this month where that the 31st Dec doesn't not actually feature in my list, so I need something that will look for the MAX date in that previous month and use that instead.

any ideas please?

that seems to work better!

thanks