Nebel's got it covered. And, like both of you, I suspect that in-universe there are many different calendars being used. On the bright side, that means we're all pretty free to make those up, since we've got nothing canon to use.

SW:TOR is just a bit vaguer than it could be when it comes to these things. I really wish they'd thrown in more backgroundy stuff. We do get fragments here and there, but a whole lot of things that would be very nice to know seem not to be found anywhere.

I suppose we could try to gather what background info the game does give us here somewhere. I don't know of any other repository of SW:TOR background data.