Azeroth, conveniently, has the same calendar as Earth, with 365 days, a seven-day week, and twelve months. Years are counted forward from Year 0, the year that the orcs invaded through the Dark Portal and changed the world forever. To find out more about the various holidays celebrated throughout the year, visit Blizzard's official In-Game Events page and click on a link to learn more about a holiday.
There were three wars in recent history. The First War was around thirty years ago and mostly concerned orcs and humans on the Eastern Continent. The Second War was a few years after that and involved dwarves, High Elves, and a few jungle trolls as well, but was still restricted to the Eastern Continent. The most recent war, the Third War, took place a little less than a decade ago, and had cataclysmic worldwide effects. Everyone on Azeroth remembers it and was touched by it in some way.
The +5 Error - in the WoW RPG manual, a bewildering 5 years of nothing is added into the timeline between Thrall's escape from Blackmoore and Thrall's joining forces with Doomhammer to free the orcs from the internment camps. The otherwise amazing fansite and lore resource Blizzplanet seems to have adopted this mistake into its own timeline, but this conflicts with the timeline on the Official WoW site, which DP uses.
Permayears - Blizzard would have us believe that in the World of Warcraft, time only advances when a new expansion pack comes out. Those of us who have been roleplaying, celebrating birthdays, marrying and having babies in their world since 2004 know better! While the world situation seems to stay remarkably similar (Dread Pirate Van Cleef, anyone?), time does indeed pass, as clocks and holidays show us. Therefore, any in-game references to how many years ago something happened should be taken with a grain of salt. For the purposes of dating journals, letters, etc., DP continues to count the years forward at the regular pace and change them with each passing January first.
Start-of-WoW Retcon: Because three years and two months have actually passed in real time from the start of WoW to the launch of Burning Crusade (supposedly only a year apart on the timeline), we did a small "retcon" to change the in-character events of the WoW launch year in our characters' diaries and memories from Year 25 to "late Year 22." Since the launch of WoW doesn't represent a specific IC historical event the way the launch of Burning Crusade does (Exodar crash etc.), we figured this was the easiest adjustment to make.
*NOTE* You are of course free to imagine time and its passage in whatever way you like for your own stories, but if you'd like to be on the same page chronologically with a large number of DP members or get intricately involved in any of our storylines, it may be easiest to "borrow" our use of time & calendar for that purpose.