After the humans will wander thru space ( at some time - very probable), there will no time reference, but only time intervals ( like, day on a spaceship has 24h , etc ). So in this case, you would measure 86400 seconds and call it a new day. No more leap seconds, etc.
Now the UNIX makes sense: count seconds since a certain event in time and meajure from there on, internally. Want to display it? Then use special computation to render it in the format ( read timezone, add relativistic skew, etc ).