I would assume each planet has it's own way of tracking time due to varying lengths of day and planetary orbits around their suns.
And then I would assume there's some kind of unified dating system. (Example Star Date: etc etc.)
I notice on the timeline in the holonet the entries are BTC and ATC. (Before/After The Treaty of Couruscant.) So we have evidence of some kind of yearly system, but no idea about how long this is relative to a year in RL.
I guess all we can say with certainty at this point is there is 456 months per year in SWTOR.
Since we can travel at light speed it also makes it more possible to have a universal date