Given 𝑛, solves for 𝑥 in the equation
1970 + 𝑛 milliseconds = 𝑥
where 1970 refers to 1970-01-01T00:00+00:00.
In other words,
console
$ unmillis -10
1969-12-31T23:59:59.990+00:00
$ unmillis 1000
1970-01-01T00:00:01+00:00
Binaries can be downloaded from Releases