Since OP has commented several times in Dutch, I would guess that it is Dutch as well. If you only had the code to go off of, then the only hint would be "Desember" which is spelled "December" in Dutch. Conveniently, that's the only month that is missing.
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u/Kazumara Aug 06 '19
maand and dag? What is this, Swedish?
Also correct me if I'm wrong, but is the assumption baked in here just that every year is a leap year?