Please pardon this drive-by assessment of JS calendar libraries by a casual user.

moment.js: Mutable. Thank you, next.

Luxon: Takes the effort to implement `Interval`, which would be `Range` in any proper language, but somehow avoids providing separate `Date` and `Time` objects.

Day.js: When you kinda like moment.js, but your bundler says it's too fat.

date-fns: The finest of pure, curry-able functions over the minefield that is Javascript's `Date` object.

js-joda: If Javascript didn't have classes already, this project would probably port the entire Java runtime to JS just to replicate them. Likely the most correct handling of date/time stuff available for the browser, but damn, at what cost?

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Edit: this turned out way too negative, my bad. All I wanted was a library that offered:

0) Type definitions.

1) Immutable classes of `Date` (year, month, day), `Time` (hour, minute, etc), `DateTime` (the prior two combined), `Instant` (for a certain moment on the global timeline), and `Duration`.

1b) `DateTime` should probably be split into two separate things, one of which is aware of time zones.

2) All the obvious date arithmetic functions - duration between dates, adding/subtracting durations, etc etc.