The odds of finding a complete and accurate timeline of the Warhammer 40,000 universe are pretty slim, depending on what degree of granularity you are actually looking for. There is simply so much available time and space for authors to play with, and each and every codex, background book, novel, or roleplaying supplement is going to expand upon or contradict the pre-existing framework.
And the best part? The nature of the warp can mean that contradictory accounts of the timeline might actually both be correct. In-universe knowledge of the past is sketchy at best - events might be surpressed, histories re-written or simply forgotten, and the veracity of even the well established "dates" might not really be as solid as we the readers might want them to be; we are talking about the Imperium of Man here after all, where ignorance is praised as a virtue.
But since that isn't a particularly helpful answer, here are links to three PDF documents that present a very detailed history... of certain specific sub-sets of the Imperium. The more well-known events of the greater Imperium may appear on these timelines mind you, but they tend to be there simply to offer perspective on the real focus, rather than as entries given equal weight in a chronicle of the years. But hey, all this stuff is official[footnote]All Fantasy Flight Games Warhammer 40,000 material has to be approved by the loremasters at Games Workshop prior to publication, so any background they might develop is therefore canonical; so sayeth Alan Merrett. Whee![/footnote] and interesting!
A history of the Calixis Sector [http://www.fantasyflightgames.com/ffg_content/dark-heresy/pdf/timeline.pdf].
A history of the Koronus Expanse [http://www.fantasyflightgames.com/ffg_content/rogue-trader/news/timeline/rogue-trader-epoch-koronus-low%20res.pdf].
A history of the Jericho Reach [http://www.fantasyflightgames.com/ffg_content/deathwatch/minisite/support/deathwatch-know-no-fear-lowres.pdf].
[small]Note that the history of the Calixis Sector is a rather less visually appealing document, presented in a no-nonsense series of bullet points rendered in spartan black and white. The other two histories are what one should expect from a FFG 40K publication though[footnote]By which I mean they put they put Games Workshop's own first party source books to shame. Seriously, FFG books are gorgeous works of art.[/footnote].[/small]