[echlebek]

That's actually not true if you're using SI units for seconds.

"The SI unit of time is the second (s): The second is the duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom."

I'm not a physicist, but I think that's a universal definition - the rate of the periods should be constant everywhere in the universe.

So universal seconds are actually very easy to nail down. Dates are always a little more troublesome. Perhaps seconds since the start of the big bang would be a possibility.

[twodave]

The point wasn't to explain how we measure it or whether it is easy or not, the point is that the unit of measure was selected for its meaning, and that meaning has little relevance on other planets.

[wbl]

You are forgetting relativistic effects.

[dfox]

Because SI itself assumes that you and whatever you are measuring is in same relativistic frame of reference. When you start to take relativity into account the whole system breaks down horribly.