At least, we have a well-defined second. Assuming that humans make interstellar travel and/or colonization happen before being visited first, it's not entirely

At least, we have a well-defined second. Assuming that humans make interstellar travel and/or colonization happen before being visited first, it's not entirely unreasonable that space-borne vessels will maintain the time and calendar system developed on Earth. It's convention, after all (we fudge it _just a bit_ here on Earth, too). Perhaps extrasolar colonies will have to develop a system that makes sense for whatever planet or moon they settle on, at which point they'd be converting between Earth time and local time for correspondence.