I'd say... whatever you choose to do, make it internally consistent.
For me, if I see dates that are obviously real-world in a fantasy world, it throws me off. Like, why does a world that didn't have the Emperor Augustus have the month of August? Why does a world without vikings have Tuesdays?
Taking your year-numbers from a significant event is a useful one - everybody does it, whether the date is static (2016 AD or 1483 AH) or whether it's counted from the accession of the current monarch (in this the 63rd year of the Reign of Our Sovereign Majesty Queen Elizabeth II).
It doesn't have to be complicated unless you want it to be, but providing people with a key will help a lot.
It's also worth noting that most people don't think about what year it is an awful lot (unless they have a job that involves writing a lot of dates) - what's more important is whether it's Monday or Tuesday, or whether it's 10am or lunchtime. The importance of dates will also vary with your setting - if you're pre-clocks, or pre-everybody-having-a-clock, numbers become even less relevant. If you work on a farm, it's not terribly important what the actual time is.
So, how important are dates to your story? If people are a bit hazy on the details of exactly when everything happens, does it really matter?
On the other hand, this can be a bit of a rabbit-hole. How far do you go? Once you've conquered dates, what do you do about weights and measures? If you start making everything up new, it can get so complicated that the reader is in danger of spending more time trying to work out what time it is and how far away things are than actually concentrating on the story.