Imperial Dating System
The Imperium has developed its own method of recording dates, which needs a bit of explanation. Most importantly, the years are "Anno Domini", 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, with 0 being the most accurate and 9 being the least. Class 0 or 1 refers to events that occurred in the Sol system, with each increasing number being a further degree of separation from Terra.
For check numbers 0-1 means that the event happened on Terra or in 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.
7, 8, and 9 class sources are somewhat subjective.
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.
This is the year within the millennium, running from 000-999. For general dates, often only the year and millennium (below) are included.
This is the millennium in which the event occurred. Most "recent" events in the Warhammer 40K universe occur in M41, or the 41st millennium. An event in the present (as of 2013) would have occurred in M3.