One of my least favorite parts of coding is working with timezones, date/time formatting and localization... Momentjs solves all this and more.
One of my favorite features is its inbuilt "humanization" of duration (and customizable thresholds) like "a day ago" or "in 5 minutes."
It has powerful parsing, amazing abbreviation support and awesome locales.
I could not recommend this more.
This library is worth it. I use it on Haptime.in (both backend in node and frontend in js) and without it I would be lost :)
Probably one of the best libs in the world.
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Haptime
VC Puzzle