On Imperial horology

Third Week, Second Month: I don't know how the priest-emissaries of the Sky Father reckon time, but it doesn't fit the sun as it rises, nor the moons as they chase it. The Tax House of the Peculor Majoris is opens and closes at weird hours, sometimes it'll even close for 'nights' at Midday. And they seem to be getting real antsy about the new year, like it was just around the corner 'stead of more'n half a year away...

Since I'm aware there are some DH players and GMs who have little to no prior knowledge of 40k, and I didn't see the usual section in the core book explaining it, 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:

• a check number of 0 or 1 indicates the date refers to an event occurring in the Sol system
  
  
• 2 indicating 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
  
  
• In contact with a class 3, but not a class 0/1 or 2
  
  
• In contact with a class 4 source
  
  
• 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
  
  
• As class 6, but the unreferenced period is between 1 and 10 standard years
  
  
• A check number of 8 indicates that the unsourced preiod is greater than 10 years
  
  
• 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; and to my knowledge this is 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, imho) that gives us a year fraction (call it a 'Day') 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 AD; or CE, depending on whether or not you prefer 'Year of our Lord' or 'Common Era').
Incidentally, this is the same year as the official setting for Dark Heresy.


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 AD, in a system that was in contact with a system in contact with a system in contact with Earth.



Now, there aren't any standardised weeks or months in the Imperial calendar (at least, not to my knowledge). Local cultures (ranging from a particular demographic/nation on one world to maybe even a whole sector, with the mode being a system) could very easily have a common definition of week and month, although the larger a particular local culture (or rather: the more widespread it is) the less such a thing will necessarily make sense.

That said, I'm tempted to (unofficially) define an Imperial standard week as ten Days and a month as 100 Days, which rather neatly gives us ten equal months of 36.5 Terran sidereal days, allows us to do neat shorthand regarding the week and month (3rd week, 4th month, 815.m41) and (if we shorten each month to 36 days, or 99 Days) we can have 5 intercalary days to play with, although that probably wouldn't work for the Imperial Day.



Local calendars will vary from the official Imperial calendar, but provided you know how long each day and year is compared to a Day and one Terran year, it doesn't particularly matter. Stuff can be swapped around as you want. Just stay consistent.