Thanks for the quick answer!

I still get the " Formula you typed contains an error"-message
I did try something similar yesterday, but every time I seem to get this
error message referring to ,MONTH part of the formula.

Any idea why this is & what would help?

" One way:
=DAY(DATE(YEAR("1-"&A1&"-"&B1),MONTH("1-"&A1&"-"&B1)+1,0))
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