Time is based on an Event. We are in the year 2010 because someone inaccurately took the birth of Jesus Christ as the base (hence b.c. And a.d. denominations). Other cultures have other years, I believe either the China or the Arabian countries have a completely different year.
In the Star Wars Extended Universe, the battle of Yavin is the base for their calendar. That was the destruction of the first Death Star, and you will find dates given as "4000 BBY" (Before battle of Yavin) or "130 ABY" (after battle of Yavin)
A calendar is only there to put events into perspective, and mainly for the reader. How long is a year? Most fictional works still use the earth year, or the character says something like "it happened 8000 years ago,in your culture that's 200 earth years"
Sometimes you don't need to have an absolute year. It doesn't matter if you are in the year 2000 or 5128, because it's just a number. Maybe you just start telling your story and when you refer to past/future events, you just use relative time: He was born 200 earth years before he met the girl of his dreams. The galactic elections will be in 2 months from now, and the elected chancellor will be in office for 5 years. With this description you just set an "epoch": a man and woman meet. All other dates are relative to this point in time, wherever it is.
Basically, you can just make up a number. Pick an event, either one that happens in the story or one that has/had a great impact on the fictional universe. You don't have to give that date to the reader if you don't want to, but it will help you greatly to keep consistency, otherwise you will encounter that a character is too old or that an event happened too long ago to be witnessed by your character. Having a calendar helps greatly.
Some Universes did fine without a consistent calendar.Star Trek's Star Date for example is completely inconsistent between the various incarnations and doesn't translate. Most people don't care, I think it's sloppy.
So my recommendation: pick an event as your epoch (year 0), plan all other events against this one, and decide if you want to disclose it to the reader or if you just want to give relative dates based on that event. You can have both options of course: year 0 is when the war ended and the first chancellor reunited the tribes, but your story happens 500 years later when your character meets his future wife when he tries to rob her store, and all events you describe in the book are relative to the failed robbery.