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Your title is very close to the name of a well-known module for working with dates. I can also tell you that what you want is as easy (or Ez) as falling off a log.
This site offers all sorts of help for students. If you don't find something useful in Tutorials, try Super Search.
By rote learning.
Via Genetic memory.
It's provided by my firmware.
I just remember them.
Thirty days hath September
My computer gets it right, usually.
I just ask someone else.
Someone punches me on the first of the month.
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