Timeline

Timeline of The Galaxy:

Ancient history
M1 - The Age of Terra (also called the Golden Age) begins.
M2
M3
M4
M5
M6
M7
M8
M9
M10
M11
M12
M13
M14
M15 - Beginning of the Dark Age of Technology^[1]
M16
M17
M18
M19
M20
M21 - Beginning of the Dark Age of Technology^[2]
M22
M23
M24
M25 - End of the Dark Age of Technology and beginning of the Age of Strife.^[1]
M26 - End of the Dark Age of Technology and beginning of the Age of Strife.^[2]
M27
M28
M29
M30 - The Fall of the Eldar, the birth of Slaanesh, and the formation of the Eye of Terror occurs. The Ages of Strife ends. The Unification Wars occur. The Great Crusade begins.
M31 - The Great Crusade ends. The Horus Heresy occurs. The first Age of the Imperium era, the Age of Rebirth, begins.
M32 - The Age of Rebirth ends. The Forging begins.
M33
M34
M35 - The Forging ends. The Nova Terra Interregnum begins.
M36 - The Nova Terra Interregnum ends. The Age of Apostasy begins.
M37 - The Age of Apostasy ends. The Age of Redemption begins.
M38 - The Age of Redemption ends.
M39 - The Waning begins.
M40
M41 - The Waning ends. The Time of Ending occurs. The Great Rift is formed. The Age of the Dark Imperium begins.
M42

Sources