To be fair JS stated that his way was not an exact science but more of an average.
Have a look at this, I haven’t used it, but I think it will do what you require:-
https://bobj-board.org/t/73436
As stated before you are best creating a calendar table, there are plenty of examples on the web - Google is your friend 
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Depending on the different date attributes required by your business will obvoiusly drive the code requirement. Also dependent will be the start from and to date, how far do you need to look back and how far in future do you wish to look forward?
We have things like, Calendar Date, Weekend Date, Quarter, Year, Week Number in Year, Day Number of Year, Day of Week(Char), Month Name(Char) Epoch Days, Months, Weeks, Years Financial Year, Quarter, Month…e.t.c.
Get one set up, you won’t regret it… 8) .
Mak 1 
(BOB member since 2005-01-06)