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Imperial Dating System

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The Imperium has two distinct dating systems.

Pre-Great Rift

The Imperium had developed its own method of recording dates, which needs a bit of explanation. Most importantly, the years are based on the "Anno Domini" system, 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:

0 123 456 M41
Check Year Fraction Year Millennium

Check Number

The check number refers to the accuracy in the date as compared to time on Terra, with 0 being the most accurate and 9 being the least. Class 0 refers to events that occurred on Terra, with each increasing number being a further degree of separation from Terra.

Location Based

A 0 means that the event occurred on Terra.

A 1 means that the event occurred within the Sol system.

Contact Based

A 2 means that the event occurred while someone present for the event was in direct psychic contact with Terra or the Sol system.

A 3 means that an individual or organization present was in psychic contact with a 2 source while the event occurred.

A 4 means that the individual or organization was in contact with a 3 OR 2 source.

A 5 means that the individual or organization was in contact with a 4 source.

A 6 means that the individual or organization was in contact with a 5 source.

Estimation Based

A 7 means that the event in question occurred within 10 years of the date listed in the rest of the Imperial date.

An 8 means that the event occurred within 20 years of the date.

A 9 class source is special. A 9-class source is an approximated date, and is usually used when recording a date within Warp travel or while on a planet that does not use the Imperial system.

Year Fraction

For record-keeping, each year is divided into 1000 equal parts, numbered 000-999. Note that this is not generally used by Imperial citizens, but is simply for administrative use. Each increment of the year fraction corresponds to approximately eight hours and forty-five minutes Terran standard.

How to determine the Year Fraction on Terra

Rather than dividing the year into approximately 365 days like we do, the Imperium divides the year into 1000 fractions. So, "549" indicates the 549th part of 1000 parts of the year.

To calculate this, first you need to find on which day of the year your event occurred. Using July 18th of 2005 as an example, you then determine the Julian date, which is the 200th day of 2005. Next we need to split it into hours. We do that by multiplying 200 by 24 then add the hour of the day the event occurred. If the event took place at 4pm (the 16th hour) the formula to find the Determined Hour would be:
200 x 24 + 16 = 4816 Determined Hour

To convert that to an Imperial Fraction, multiply your Determined Hour above by the Makr Constant: 0.11407955.
4816 x 0.11407955 = 549.41. Always round this number down to determine the closest Year Fraction that occurred before the event. So the Year Fraction would be 549.

Year

This is the year within the millennium, running from 000-999. For general dates, often only the year and Millennium (below) are included. If the Terran calendar year was 2005 then the Imperial Dating System Year would be 005.

Millennium

This is the millennium in which the event occurred. Most "recent" events in the Warhammer 40K universe occur in M41, or the 41st millennium. If an event was in 2005 it would have occurred in M3 or M03, the 3rd millennium.

Post-Great Rift

After the formation of the Great Rift in the ending stages of the 41st Millennium, the flow of time no longer became standard across the galaxy. As a result, Roboute Guilliman attempted to amend the dating system and create a reliable Imperial history to fit the new situation. The result has been met with controversy and a civil war known as the Chronostrife has broken out within the Inquisition's Ordo Chronos.[2]

The new Imperial dating system takes into account the local flow of time, which usually does not match the standard elsewhere.[3]

Vigilus Template

The method of the new system seen below uses the world of Vigilus as a case study.[3]

The first element of Vigilus' timestamp is the annual designator. It starts with the number of years either before or after the rift opened, and then a number of chronosegments within that year as the second element. Imperial days are broken down into choronosegments of eight hours. After this is a third element - either 'previo' if the events occurred before the opening of the Great Rift, or 'post' if after it. This third element is sometimes denoted as a minus sign or plus sign.

The fourth element of the Vigilus timestamp is the system's designator initials; in essence, the initials of the system to which it refers. For Vigilus, this is VCM which stands for "Vigilus Cicatrix Maledictum". For its neighboring planet, Omis-Prion, the designator initials would be OPCM.

By way of a full example, if the time of an event in the Vigilus System was three days (nine chronosegments) before the opening of the Great Rift, the timestamp would be '0.9 previo VCM.M41', also expressed as '0.9 previo' or '0.9-'.

This translates as '0 years and 9 chronosegments (three days) previous to the Vigilus System's first instance of the Cicatrix Maledictum in the 41st Millennium.'

If it was one year and eight hours after, the timestamp would be '1.1 post VCM.M41', also expressed as '1.1 post' or '1.1+'.

Sources