How to get 13 Digit Epoch time in Shell (Ruby Version)

I needed this, because Dendron uses 13-digit epoch time, for created and updated timestamps. While it is not a problem when notes are created from within VSCode (as is expected - since Dendron is a VSCode plugin) - I don’t use VSCode much. While Dendron has command line tools to create new notes, and notes are markdown only the timestamps are problem. Hence, the need to create 13-digit epoch time.