There's a great question fluttering around the community lately: just when are we anyway? It's a good one for the Lore Nerd, actually, and maybe he can answer it in more detail than I can, but it's almost a question of relativity as much as it is of lore. The official lore says that World of Warcraft takes place four years after Warcraft III (hence the reason for the "four years have passed..." in the opening cinematic). And since then, we've heard from Blizzard that they consider every expansion to be another year in the history of Azeroth -- Burning Crusade takes place a year after the original game, and Wrath of the Lich King two years after that. But when, for example, did Van Cleef fall? Has it been just under two years since his plan to attack Stormwind was defeated? And what about our characters -- you'd assume that if you started from level 1 today, the story would be beginning four years after the events of Warcraft III again, so have our characters aged two years since they first stepped foot out of Elwynn or Durotar? And we'll ignore that when you get to Orgrimmar, you might see Death Knights walking around, who technically won't join the Horde until two years in the future.
See how confusing? Of course, the right answer to all of these questions is "it's just a game," but there are plenty of interesting thought experiments here. Time and story in the World of Warcraft aren't necessarily connected, which is why Blizzard can get away with things like having Kael'thas in two places at once, but still tripped up by things like getting Onyxia out of Stormwind. Phasing helps a little bit (and who knows what WoW would be like if Blizzard designed it from the ground up with phasing -- we'll have to wait for the next-gen MMO to see how that works). The official timeline has Arthas brooding in Icecrown for around six years. But just like real relativity, our characters have experienced that time period in all sorts of different ways.