The Chronostrife is a bitter, ongoing internal conflict within the Ordo Chronos over the Imperium of Man's standard dating system that occurred during the first century of the 42nd Millennium. Sometime after his resurrection in 999.M41, Roboute Guilliman, the Lord Commander of the Imperium and Primarch of the Ultramarines Chapter, was intent on making a full and accurate accounting of the Imperium's fragmentary and often contradictory history, the truth of which would inevitably help humanity's advancement out of the darkness that had befallen it in the millennia after the Horus Heresy.
For this task, Guilliman formed the small Imperial organisation known as the Logos Historica Verita, formed from individuals of an inquisitive and liberal nature. After training them personally in objective methods of data collection, the Primarch assigned them 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.
But the monumental task he had set before them was made all the more daunting when the Primarch discovered that even the Emperor's calendar had not survived the millennia intact. During the Great Crusade and the Horus Heresy, the standard Imperial 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 human-settled galaxy almost impossible to construct. As the Indomitus Crusade drew to a close sometime 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, 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.
- Dark Imperium (Novel) by Guy Haley, Ch. 9 "Imperator Gloriana," pp. 96-97
- Imperium Nihilus - Vigilus Defiant (8th Edition), pp. 10, 20