The March of Time by Rick Priestley (White Dwarf 97)
THE MARCH OF TIME
The Imperial dating system is based upon the old calendar - the one with which we are all familiar. An Imperial date is therefore a date 'Anno Domini' but it's expressed in rather different terms than those we are used to. The most noticeable difference is the suffix 'M' followed by a number. This is the millennium number: Ml is the first millennium and so on. We are living in 1987 which is the second millennium. In Imperial terms any date between 1001 and 2000 would be suffixed by M2. The current millennium in the WH40k mythos is the forty-first or M41. Incidentally, this suffice is normally emphasised by a full stop (or oblique if you prefer) for clarity.
A typical dating code, such as you will find in the WH40k book and in WD articles, is 0150935/M32. The M32 means we are dealing with a thirty-second millennium date. The other numbers tell us the year and fraction of the year.
0 (check number ) 150 (year fraction number) 935 (year number) M 32 (millennium)
Check number The first digit in the sequence is the dating reference or check number. This check number is necessary due to temporal distortions which affect ships in warp-space as well as worlds which are remote, or isolated, from Earth. It's presence qualifies the accuracy of the date given in each case.
Prefixes 1 to 8 indicate gradually widening 'grey areas' surrounding the origins of a given item of data. Prefix 9 is slightly different. It's used when, for instance, a source reporting from a world that doesn't use Imperial dating, needs to make a reference to that world's history. The historical date would have to carry the prefix 9.
Year The last three digits are the year within the millennium running from 001-000 (one thousand). For example 0150930/M32 is the year 930 of the thirty second millennium. We would describe this as the year 31930 AD. When referring to a year in general terms, and where it is not necessary to include the year fraction or check number, it is acceptable to write 'year 930/M32'.
Year fraction For administrative purposes the standard year is divided into 1000 equal segments; 001-000. This is a purely administrative convention and not a part of everyday usage.
The following examples should make this clear.
0333042. M32. Segment 333. Year 42. Millennium 32. Our year 31042
4590640. M41. Segment 590. Our year 40640.
0001987. M41. Segment 1. Our year 40987 and the 'current' year in the WH40k universe.
As the last example explains, the current year in the WH40k mythos is year 987/M41. The current 'real' real is, of course, year 987/M2. Because it makes the game easier to write for, I usually refer dates in the WH40k mythos to the approximate 1987 equivalent at the time of writing. Obviously it is not possible to coordinate 'game time' and 'real time' absolutely, but it does add coherency to a campaign structure. Your campaigns may be developed in the same way, but feel free to be flexible. If you command a force which must travel through warp space for six months of game time, it's hardly reasonable to wait six months before fighting the battle!