Can you calculate unix timestamps (universal clock) from any planet? Given the definition of unix timestamp as the number of seconds elapsed since January 1st, 1970 as GMT+0, without leap seconds, is it possible to create a universal clock that will generate the correct timestamp?
Is the current definition of a second 

the duration of 9192631770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom.[1] In 1997 CIPM added that the periods would be defined for a caesium atom at rest, and approaching the theoretical temperature of absolute zero (0 K), and in 1999, it included corrections from ambient radiation.[1] Absolute zero implies no movement, and therefore zero external radiation effects (i.e., zero local electric and magnetic fields).

sufficient for this?
 A: Yes it is certainly possible to construct a universal clock.
If you are at rest with respect to the average matter in the universe (basically this means being at rest with respect to the cosmic microwave background), and not in a gravitational field, then you are a comoving observer. Every comoving observer will agree on the time since the Big Bang (13.798 $\pm$ 0.037 billion years), and they will all agree on the rate that time passes i.e. the length of the second (defined using your caesium standard). So we can use the number of seconds since the Big Bang as a universal clock that applies everywhere in the universe.
This isn't a particularly practical clock, since we have little hope of every knowing the age of the universe to an accuracy comparable to one second. Still, in principle it could be done.
If you get in a spaceship and fly around the universe then your local clock will get out of sync with the universal clocks due to relativistic time dilation, but you can in principle calculate the loss of sync and correct your clock as you go. Likewise if you're in a gravitational field your local clock will run slower than the universal clock, but again this can in principle be corrected.
