I've seen too many broken implementation of leap years...

Me too. Speaking of which, what should !$year%4 && ($year%100 || $year%400) do? if a number is divisible by 400, it is always also divisible by 100. (Update: not sure if that sentence makes sense. Other way round: if it's not divisible by 400 it's also not divisible by 100. Does that make more sense?)

My suggestion is not to re-invent the wheel, and work around the "no external module" limitation. I quite like Date::Simple module which does about everything you want for date processing (if you don't need times), and is very intuitive.

I'd construct a leap year condition like that, but I might be wrong - which is why I used a module:

my $is_leep = ($year % 400 == 0) || (($year % 4 == 0) && $year % 100 !
+= 0))
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