This is the year within the millennium, running from 001-999. 2018 would be represented very generally in the Imperial Dating System as 018. 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.
For general dates, often only the year and millennium are included, such as 999. This is the millennium since the birth of the ancient human religious figure Jesus Christ in which the event took place. Not long after the Ultramarines Primarch Roboute Guilliman's resurrection in 999.
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.
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.
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.The Cicatrix Maledictum became the defining point of the new era. 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.