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 writes 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 occurred 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 data 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 practised, 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 practised 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.