Doomsday Algorithm: Fix leap year check

[minh-swinburne]

Replace != in (year % 400) != 0 (line 49) with ==

Justification: Years that are divisible by 100 (centurian == 100) but not by 400 (year % 400 != 0) are skipped and NOT leap year.

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[imSanko]

Parenthesize a and b expressions when chaining and and or together, to make the precedence clear

[minh-swinburne]

I've changed the incorrect conditional operator and ensure clarity using parentheses