Imperial Dating

Interpreting Dates in the Imperium of Man

Since Rouge Trader, Warhammer 40K has used an arcane dating system.  I thought I'd do the obligatory explanation of horology in the 41st millennium, and make sure people can figure out some of the weird dates.

A typical date might be 4.423815.M41. This can be broken down into 4 basic parts- 4.  423   815  M41.
The first part is the check number, which determines the degree of probable accuracy in the rest of the date:

0 indicates an event occurring on Earth

1 indicates the date refers to an event occurring in the Sol system

2 indicate a system in direct astropathic contact with Terra

3 indicates the source was not in direct contact with Terra, but was in contact with a class 2 source

4 indicates a source in contact with a class 3, but not a class 0/1 or 2

5 indicates a source in contact with a class 4 source

6 indicates a source that was not in direct psychic contact with any other source at that time, but the date is part of a sequence that begins or ends with a source referenced date of class 1-5. The unreferenced period is no greater than 1 standard year

7 indicates a source similar to a class 6, but the unreferenced period is between 1 and 10 standard years

8 indicates a that the unsourced period is greater than 10 years

9 indicates an approximated date- either this belongs to a date sequence with no fixed co-ordinate at either end (such as a date given from the Dark Age of Technology), or the date has been approximated from a non-imperial dating system

Obviously, check numbers of 0-5 are relatively accurate, while 6-8 represent a widening grey area of inaccuracy. Class 9 is somewhat different, but can generally be assumed to be roughly accurate (maybe). It is probably worth remembering that the period separating the check number from the rest of the date is optional.

The next portion of three digits (423 in this case) is the year fraction. Each year is broken up into 1000 equal segments, numbered 000-999, for administrative purposes (I suspect; though not stated in canonical fluff, but is fairly logical to assume; that these 'administrative purposes' are to make it easier to convert local dates, it being simpler to convert something in relation to 1000 than to 365).

Assuming an Imperial standard year is based upon a Terran sidereal year (not too unreasonable) that gives us a year fraction (call it a 'watch') of just over 8hrs, 45 minutes and 57 seconds in length. Perhaps not unsurprisingly, this portion is not really in general use among Imperial citizens, although anyone who needs to refer to and keep track of time on different planets will do so in preference to local date systems. Our example date is sometime between ~11:30 AM and ~8:15 PM on May 16th, therefore.

The next three digits are the year, going from 001 to 000 (one thousand), so our example date is the eight hundred and fifteenth of its millennium, which brings us neatly to the final portion of the date, which is the millennium itself. m41 (or occasionally M41, or just /41 or .41) is the forty first millennium, the 'current' period of the game setting, meaning our example date (which, like all general dates, may be abbreviated to the form 815.m41) is the 815th year of the 41st millennium (or the year 40,815 CE). Incidentally, this is the same year as the official setting for Dark Heresy RPG and a lot of the GW fluff.
So, if we put this all together, what do we get? 4.423815.m41= between 1130 and 2016 hrs on May 16th, in the year 40,815 CE, in a system that was in contact with a system in contact with a system in contact with Earth.

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