Imperial Dating System
The Imperium has developed its own method of recording dates, which needs a bit of explanation. Most importantly, the years are based on the "Anno Domini" system, 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 0123456.M41. Let's divide this code up to explain each part:
The check number refers to the accuracy in the date as compared to time on Terra, with 0 being the most accurate and 9 being the least. Class 0 refers to events that occurred on Terra, with each increasing number being a further degree of separation from Terra.
A 0 means that the event occurred on Terra.
A 1 means that the event occurred within the Sol system.
A 2 means that the event occurred while someone present for the event was in direct psychic contact with Terra or the Sol system.
A 3 means that an individual or organization present was in psychic contact with a 2 source while the event occurred.
A 4 means that the individual or organization was in contact with a 3 OR 2 source.
A 5 means that the individual or organization was in contact with a 4 source.
A 6 means that the individual or organization was in contact with a 5 source.
A 7 means that the event in question occurred within 10 years of the date listed in the rest of the Imperial date.
An 8 means that the event occurred within 20 years of the date.
A 9 class source is special. A 9-class source is an approximated date, and is usually used when recording a date within Warp travel or while on a planet that does not use the Imperial system.
For record-keeping, each year is divided into 1000 equal parts, numbered 000-999. Note that this is not generally used by Imperial citizens, but is simply for administrative use. Each increment of the year fraction corresponds to approximately eight hours and forty-five minutes Terran standard.
How to determine the Year Fraction on Terra
Rather than dividing the year into approximately 365 days like we do, the Imperium divides the year into 1000 fractions. So, "549" indicates the 549th part of 1000 parts of the year.
To calculate this first you need to find which day of the year your event occurred on. Using July 18th of 2005 as an example, you then determine the Julian date which is the 200th day of 2005. Next we need to split it into hours. We do that by multiplying 200 by 24 then add the hour of the day the event occurred. If the event took place at 4pm (the 16th hour) the formula to find the Determined Hour would be:
200 x 24 + 16 = 4816 Determined Hour
To convert that to an Imperial Fraction, multiply your Determined Hour above by the Makr Constant: 0.11407955.
4816 x 0.11407955 = 549.41. Always round this number down to determine the closest Year Fraction that occurred before the event. So the Year Fraction would be 549.
This is the year within the millennium, running from 000-999. For general dates, often only the year and Millennium (below) are included. If the Terra calendar year was 2005 then the Imperial Dating System Year would be 005.
This is the millennium in which the event occurred. Most "recent" events in the Warhammer 40K universe occur in M41, or the 41st millennium. If an event was in 2005 it would have occurred in M3 or M03, the 3rd millennium.
Following the resurrection of Roboute Guilliman in late M41, the Primarch attempted to reform the Imperial dating system more efficiently and create a reliable Imperial history. The result has been met with controversy and a civil war known as the Chronostrife has broken out within the Inquisition's Ordo Chronos.