Milliseconds to date converter

Converts a millisecond timestamp — the thirteen-digit kind `Date.now()` returns — into a date, and into every other epoch alongside.

JavaScript is the main source of these, and the mismatch with the rest of the world is a constant source of off-by-a-thousand bugs: `Date.now()` gives milliseconds while a database `to_timestamp()`, a Python `time.time()` and most APIs give seconds. Passing one where the other is expected does not error, it just produces a date in 1970 or in the far future, which is why it survives code review. If you are storing timestamps, pick one unit and be explicit in the column name — `created_at_ms` costs nothing and prevents the whole class of bug.