[_jal]

Time, to me, is the canonical example of "things that people think should be easy, aren't". It has everything - complex, constantly changing "business" rules, exceptions generated essentially randomly by a shadowy cabal[1] nobody's ever heard of, "impossible" situations like the 11 days that never happened and other technically arbitrary calendar edits[2], multiple silly base conversions, really weird rules for picking certain dates[3], lots of opportunities for making fencepost errors, and lots of other things.

Which leads me to my rule with time programming: Never fail to use a solid library, unless you're unfortunate enough to be writing one.

This[4] is a great, necessary but not sufficient book if you have to do that.

[1] https://en.wikipedia.org/wiki/International_Earth_Rotation_a...

[2] For example, https://en.wikipedia.org/wiki/Swedish_calendar

[3] https://en.wikipedia.org/wiki/Easter#Computations

[4] https://www.amazon.com/Calendrical-Calculations-Nachum-Dersh...

[deepsun]

Don't even mention how to calculate number of weeks in a year! It's often needed in real world programming for business reports, yet Joda-time library still doesn't support weeks not starting on Monday (like USA-ones) for that.