Change Date from "DD.MM.YYYY" to "MM.YYYY"

Dear Members,

I have the following problem in EXCEL. I have two lists of data, both as a time-series, meaning the data within the list is linked to a date. I now want to match some Data of the two lists by using XLOOKUP and the Date. The problem is, that it is monthly data but the date for the same data is different (for January 1965 I have 31.01.1965 and 30.01.1965). Therefore, XLOOKUP does not work. I already tried quite a lot of different approaches but none of them converted the two Dates into identical monthy dates so I could apply XLOOKUP.
Does anyone know how to convert these different Dates into the same "Value"/monthly dates? The final format of the date is not important, whether it is "January 1965", "01-1965", "1965-01" or something else.

Thank you all very much in advance.

Sincerely
Dennis