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 record-keeping, each year is divided into 1000 equal parts, numbered 001-000. 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 001-000. 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 2005) would have occurred in M3.