How old is Sanctuary?

Diablo is followed shortly by Diablo II and Diablo III comes 20 years after that, but how old is the world of Sanctuary as a whole?

According to some timelines, III occurs in 1285, but did the counting of the years start with the creation of Sanctuary or the winking into existence of the High Heavens/Burning Hells? I figure as angels and demons aren't mortal, they don't really count the years as us mortals do, but I haven't found anything that says this outright.

Solution 1:

There appear to be no exact dates referenced in relation to the Sin War or Great Conflict, the creation of Sanctuary happened after the Great Conflict had started. You are unlikely to find a specific date, as books were written by men, and men didn't exist until after the creation of Sanctuary.

Blizzard Entertainment has released the official timeline from 964 to 1285. However, events prior to this have never been given precise dates. As you can see from this link, everything before these dates are assumptions, and the creation of Sanctuary appears to be labelled as year one.

This would make Sanctuary 1285 years old, baring in mind that the Heavens and Hell already existed by this point.