You can't use any time for observers in different reference frames as the length of a second will be different. UTC is generally fine as far as normal time goes

You can't use any time for observers in different reference frames as the length of a second will be different. UTC is generally fine as far as normal time goes as we're all in roughly the same reference frame on Earth, and indeed anywhere in the Solar System. It would still drift by noticeable milliseconds per year, but we can cope with leap seconds just fine with UTC and that's a larger drift. You'd probably want to avoid leap seconds and use TAI though, but we should avoid leap seconds anyway IMO.