If humans one day conquer all of our solar system we might have settlements not just on Mars and Earth, but also in orbit of Venus or Mercury, and on the moons of the gas planets, like Jupiter's Europa. Each of those has a different length of day and year.

How would an interplanetary civilization find a common date and time system?

I assume each planet gets it's own local time, but I think a common date-time system would be usefull.

Also I don't like to just say, earth is human kinds origin, so it's time is the standard. Feels lazy.