I found this at http://www.gamehobby.net/subject_indexes/subject_wh40k_chapter_approved.html and thought it was interesting. Unfortunately, my collection of WD's start at issue 99...

Chapter Approved
The March of Time: Imperial Dating System

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 suffix 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.
#
0/1 Earth standard date. Referring to an event which happened within the Sol system.
2 Direct. Source in direct psychic contact with Earth when date reference was made
3 Indirect. The source is in direct psychic contact with a class 2 source but not Earth.
4 Corroborated. The source is in direct psychic contact with a class 3 source but not a class 0/1 or 2.
5 Sub-corroborated. The source is in direct psychic contact with any corroborated source.
6 Non-referenced 1 year. The source is not in psychic contact with a class 1-5 source when the reference is made. However, the date belongs to a sequence begining or ending with a date which does have a class 1-5 source reference. The unsourced time period is no greater than 1 standard year.
7 Non-referenced 10 years. This is an unsourced date in the same way as a class 6 date but with an unsourced period of 1-10 years.
8 Non-referenced more than 10 years. This is an unsourced date as for 7 but for an unsourced sequence of more than ten years.
9 An approximated date. A date with no fixed coordinates at either end of a sequence, or a date approximated from non-Imperially dated references.

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!

I remember this, now. Well dug up, I'd forgotten that there is a direct translation of the cryptic Imperial date codes. Nice one. :¬)

I actually ruled in my DH game that there were, in fact, two calendars.

One that used the Administratum's thousand-segment year, and one that actually used 24-hour days and 365-day years out of reverence for the cycles of Holy Terra.

I had the latter used by normal people in everyday dealings, so today would be 21.010.M3 in short form.

The fact that the Administratum used a different calendar that makes absolutely no sense whatsoever to anyone who isn't one of them added another layer of bureacracy to have to deal with, and the group's Adept was the only one who could convert between the two without having to rely on a cogitator engine. The Tech Priest didn't count because that's exactly what he did.

Aye, the common people are unlikely to even pay attention to the full year, probably something more like the historical Japanese or Chinese calendars "The Year of the Lotus Blossom" or whatever. ;¬)

I think, in reality, planetbound people are going to use a calendar specific to that planet, out of necessity, even if specific just means how many extra days they have to add at the end of a year to match a theoretical calendar suited to another planet (Holy Terra seems fine by me). Timekeeping, on a daily basis, runs into the same problem, too - local dawn is when the sun comes up and the time between that and a repeat of it is a local day, like it or not. The dual calendar adding obscurity is very 40K Administratum, though, isn't it just? :¬)

Gaidheal said:

There would be some planets/places where the admin calender would be predominant, though. Those would be the places where planetary seasons and sun position no longer play a major or predominant role in the peoples lives, such as heavily industrialized hives and void ships and stations. In my personal 40kverse, warp-ships and majorly devolved hive worlds where daylight is a thing of legend for most folks worked off of the administatum's calender. They don't have day and night, they just have a cycle. Each cycle is composed of three shifts. Each shift is 1 chronosegment and, depending on your position and privileges, you either worked for two shifts and rested for one or, if lucky, worked for one shift and rested for two.

The admin calender could also hold sway on worlds which have a long rotational period where it's easer on the human system to break the long day and night cycles into more manageable chunks. Such calenders would still probably be approximations of the admin calender since they'd still need to sync up to sun rise and sun set, but they might be close with some funky leap segment system to kick them back in sync when they get too far out of alignment with the admin dates or some such.

So, for any given planet, you're likely to have three different calendars. The Administratum calendar, the local calendar (still useful for, say, farmers, who need to measure the local seasons more than anything else), and the calendar used by everyone else.

The 24/365-style calendar would work as well as anything else on a mining outpost with no crops or anything, regardless of its accuracy to local conditions (i.e., the planetoid could orbit its star every 43.8 years and have a local day/night cycle of 16 hours, but the humans tearing ore out of it wouldn't need to really know any of that to do their jobs).

Aye, all good points, borne out by my own experiences right here on Holy Terra. ;¬)

One thing I did with my DH game is break it (the Admin calendar) down a little further, decreeing that 100 "chronosegments" (I hate that term, and prefer "Day", to contrast with a "day", which would refer to the local calendar) to be a Month, with 5 Weeks of 100 Days apiece. This actually works out neatly as 10 Months of 30 days in a year, with 5 Weeks of 6 days in each Month.

Of course, the Administratum tended to ignore that distinction (officially), so the only people that used it were voidfarers and those in the ecclesiarchy that wanted to try and hold "simultaneous" ceremonies and celebrations of various Saints, and grand Imperial victories, but I figured it was pretty cool