The Imperial Dating System of the Imperium of Man, also known as the Imperial Calendar, is fairly complex in nature, and has been structured so as to deal with the vast amount of recorded history that exists in the 41st Millenium and the massive distances between settled human worlds, which can lead to long periods of isolation. Also the vagaries and time-warping effects of the Immaterium can make it almost impossible to keep accurate track of time over long journeys.
The Imperium has developed its own method of recording dates, which needs a bit of explanation. Most importantly, the years are always "Anno Domini" (AD) using the numbering system of our own present-day Gregorian Calendar, so the dates themselves are the ones that we are familiar with now. A typical date as Imperial scholars write it would look something like 0.123.456.M41. This date can be divided up so that each part is explained separately:
0 123 456 M41 = Check Number/Year Fraction/Year/Millennium
The check number refers to the accuracy of a given date, which is required for clarity due to the common distortions of subjective linear time that occur while travelling within the Warp, and inaccuracies in timing on remote or isolated Imperial worlds and star systems. Check numbers are listed using the following numeric classes:
- Class: 0/1 - Earth Standard Date. Referring to an event which happened within the Sol System. "0" is most commonly used to refer to an event which actually occured on Terra, while "1" refers to an event that occurred somewhere within the Sol System, such as Titan, the moon of the gas giant Saturn that serves as the headquarters of the Grey Knights Space Marines.
- Class: 2 - Direct. The source for the event the date records was in direct psychic contact with the Sol System or Terra when the date reference was made.
- Class: 3 - Indirect. The source for the event the date records was in direct contact with a Class 2 source, though not the Sol System itself.
- Class: 4 - Corroborated. The source for the event the date records was in contact with a Class 3 source.
- Class: 5 - Sub-Corroborated. The source for the event the date records is in direct psychic contact with a corroborated source.
- Class: 6 - Non-Referenced, 1 year. The source for the event the date records is not in contact with a Class 0 to 5 source when the date reference was made, though it is a continuation of a timeline referenced by a Class 0 to 5 source.
- Class: 7 - Non-Referenced, 10 years. As with Class 6 sources, but with a greater time span to allow for larger inaccuracies.
- Class: 8 - Non-Referenced, 11+ years. As Class 7, though unsourced for a longer period of time.
- Class: 9 - Approximation. For when worlds using a non-Imperial calendar must reference an event in that world's history.
For record-keeping, each year is divided into 1000 equal parts, numbered 001-000, rather than months or weeks. Note that this system is not generally used by Imperial citizens in everyday life, but is simply for administrative use by the Adeptus Terra.
This is the year within the millennium, running from 001-999. For general dates, often only the year and millennium are included, such as 999.M41.
This is the millennium since the birth of the ancient human religious figure Jesus Christ in which the event took place. As an example, the year A.D. 2018 would be represented very generally in the Imperial Dating System as 018.M3. As almost all records of the religions practiced by Mankind before the development of the Imperial Cult in the 31st Millennium have been lost, none know exactly why the Imperial Calendar counts the years in this manner or the true origin of the dating system.
Not long after the Ultramarines Primarch Roboute Guilliman's resurrection in 999.M41 as the Lord Commander of the Imperium, he launched the Indomitus Crusade to push back the encroaching Forces of Chaos that had been unleashed across the galaxy after the birth of the Great Rift. Even as he made war, the Primarch attempted to grasp the events of the ten millennia that had passed since he had fallen during the Battle of Thessala to the Daemon Primarch Fulgrim.
He was dismayed to find that much of humanity's history had fallen afoul of superstition, fanaticism and hearsay during his absence. If anything, the state of Mankind's knowledge was worse than it had been during the Unification Wars, when the Emperor had unified Terra before the Great Crusade. Much of Terra's ancient history, painstakingly pieced together by the Remembrancers of Guilliman's own time, had been lost again.
The Primarch was determined to make a full and accurate accounting of the Imperium's fragmentary and often contradictory history, and so he commissioned the formation of a new organsation -- the Logos Historica Verita. It was charged with the daunting task of discovering, collating and cataloging thousands of standard years of human history that had been lost or was woefully inaccurate due to superstition, suppression or purposeful obfuscation by the High Lords of Terra.
The task of compiling a full and accurate accounting of the history of humanity was a monumental and nigh-impossible task, compounded further by the fact that even the Imperial Dating System was woefully inaccurate. As the Primarch digested thousands of standard years of history recorded upon every device conceivably employed by Mankind, he had come to learn of the Chronostrife of Terra, a bitter, ongoing internal war within the Ordo Chronos over the Imperium's dating system. What he read made him despair. Not even his father's calendar had survived the millennia intact.
During the Great Crusade and the Horus Heresy, the standard dating system had provided some idea of the order of events over time, but like everything else the Emperor had created, the calendar had become degraded by both dogmatic adherence and thoughtless revisionism. Various rival dating systems had evolved from the Imperial Standard, making a true chronicle of the galaxy almost impossible to construct.
As the Indomitus Crusade drew to a close around 111.M42, Guilliman calculated the current year by the five main factional variants of the Imperial Calendar to be anywhere between the early 41st Millennium and an entire millennium later, and that was leaving out the numerous lesser, more heretical interpretations. Guilliman had been hoping to find a solution to the Imperium's tortured dating system; he had instead found there was none. It was something else that would ultimately require his personal attention to repair.
- Warhammer 40,000: Rulebook (5th Edition), pg. 127
- Dark Imperium (Novel) by Guy Haley, Ch. 9 "Imperator Gloriana," pp. 96-97