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 below:
0 123 456 M41 = Check Number/Year Fraction/Year/Millennium
Following the birth of the Great Rift, temporal anomalies spread across the galaxy making the use of a universal dating system extremely difficult as different Imperial worlds began to experience the passage of time at different subjective rates. A new, more localised dating system came into existence that was different for each world. It used the birth of the Great Rift as the standard event from which all time was calculated, either before or after its creation.
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.
The problem faced by Guilliman was that after the opening of the Great Rift, near every active Imperial war zone had to devise and reinforce its own chronological system. Even had the Imperium of Man not been split in half, the sheer interstellar distances it covered prohibited any accurate reflection of time and space.
Despite the flexibility of the "check number" system of old outlined above, pioneered at the time of the Great Crusade, where the first digit of each timestamp indicated its veracity, it became easy for dates and times to lose all meaning between star systems.
Guilliman finally resolved the problem after a long conference with his Historitors and the mysterious organisation of the Ordo Chronos. He decreed that a single logic could no longer be applied to time and space within the Imperium.
Through the High Lords of Terra he made his theory and resultant process into law. There could no longer be a unified Imperial calendar with so much temporal distortion occurring. Even though the rift's temporal warping effects might not have reached the furthest spinward planets at the galaxy's fringes, its psychic echoes -- and the lack of the Astronomican that resulted -- were still felt profoundly on those worlds.
Therefore, each sub-section of the Imperium would have to look to its own chronology, and use the coming of the Great Rift in that sector as its overarching reference point. The Cicatrix Maledictum became the defining point of the new era. This new system is outlined below.
Dark Imperium Dating System
In the wake of the birth of the Great Rift, temporal anomalies abounded across the galaxy, with the subjective flow of time on some Imperial worlds being wildly at odds with what was experienced elsewhere. This was particularly true in the Dark Imperium, the half of the galaxy that had been cut off from the light of the Astronomican and was most at risk of Warp incursions.
To deal with this reality, a new system for tracking the date came into existence on many Imperial worlds during the Age of the Dark Imperium. This system used the birth of the Great Rift as the standard event from which all time was calculated, either before or after its creation. The example given below was the version of this system which prevailed on the Sentinel World of Vigilus.
The chronology of Vigilus, as denoted in records of the War of Beasts, uses as its anchor point the opening of the Great Rift above the planet. This was an event of such incredible magnitude it rewired the planet's temporal logic altogether, much as it did a thousand other war zones across the Imperium.
The first element of Vigilus' timestamp is the annual designator. It starts with the number of Terran years either before or after the rift opened, and then a number of chronosegments within that year as the second element. Imperial solar days are broken down into chronosegments of eight solar hours. After this is a third element -- either "previo" if the events occurred before the opening of the Great Rift, or "post" if after it. This third element is sometimes denoted as a minus sign or a plus sign.
The fourth element of the Vigilus timestamp is the system's designator initials; in essence, the initials of the star system to which it refers. For Vigilus, this is VCM which stands for "Vigilus Cicatrix Maledictum." For its neighbouring planet, Omis-Prion, the designator initials would be OPCM.
By way of a full example, if the time of an event in the Vigilus System was three solar days (nine chronosegments) before the opening of the Great Rift, the timestamp would be "0.9 previo VCM.M41," also expressed simply as "0.9 previo" or "0.9–."
This translates as "0 years and 9 chronosegments (three solar days) previous to the Vigilus System's first instance of the Cicatrix Maledictum in the 41st Millennium AD."
If it was one Terran year and eight solar hours after the opening of the Great Rift, the timestamp would be "1.1 post VCM.M41," also expressed as "1.1 post" or "1.1+."
- Warhammer 40,000: Rulebook (5th Edition), pg. 127
- Dark Imperium (Novel) by Guy Haley, Ch. 9 "Imperator Gloriana," pp. 96-97
- Imperium Nihilus - Vigilus Defiant (8th Edition), pp. 10, 20