epoch prints the current time in epoch millis, or accepts an epoch time on the command-line and prints the corresponding RFC3339 time.
For example:
$ epoch
1652400221740
$ epoch 1652400221740
2022-05-13T00:03:41.740+00:00
$ epoch 2022-05-13T00:03:41.740+00:00
1652400221740
src/main.rs
is also directly runnable using rust-script