kDay, MDay, etc could work in space but they don't fit well with the length of the year. As long as we live on Earth we cannot escape from our planet taking about 365 days to orbit the Sun. History proved that it's convenient to have the same event (let's say start of spring) falling at the same date every year. Hence all the refactoring of calendars and leap years.
I wonder how we would settle that matter if we'll ever be able to travel fast between planets. Each city had its own time zone before trains required us to sync them because of conflicting railroad timetables. So we ended up with the current timezones. With planets, each one would have its day length and number of days in the year, maybe even inconstant seasons in the case of precession of perihelion or double star systems. I'd say we'd settle on local time and a common space time but who knows.
I wonder how we would settle that matter if we'll ever be able to travel fast between planets. Each city had its own time zone before trains required us to sync them because of conflicting railroad timetables. So we ended up with the current timezones. With planets, each one would have its day length and number of days in the year, maybe even inconstant seasons in the case of precession of perihelion or double star systems. I'd say we'd settle on local time and a common space time but who knows.