I think human racial differences exist. Individual differences exist, obviously, but you also can't deny that you are physically much more similar to some peopel than others. A black man will tend to have darker skin, thicker lips, blacker and curlier hair, and of course a much better sense of rhythm

(sorry, I can't resist the urge to slip a little racist joke in here...

, I apologise if anyone's offended). Certainly it might be arbirtrary to draw the line between races - nowadays we've all got a mix of different blood in us. But the same would go for dogs, really. It seems odd to deny that there are racial differences among humans when there are such obvious trends in physical characteristics of people from certain parts of the world.
Anyway, back on topic. Maybe time is the same. The way we measure time, calanders and clocks, might be fairly arbitrary, jsut as it can be quite arbitrary to call someone half chinese and half Scottish or soemthing. But nevertheless, the passage of time might be a very real thing, just as race is, I think, a real thing.