Imperial Dating System

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The original Imperial Calendar[2] also known as the Terran Standard Calendar[7] was created by the Emperor of Mankind sometime before the end of the Great Crusade. By the (so-called) 42nd millennium[Conflicting Sources] at least five "non-heretical" variations of the original calendar existed.[2] The Imperial Calendar bases its years on the Anno Domini system[Trivia], beginning at 1AD/CE. A typical year, as Imperial scholars write it, would look something like 456.M41.[1] When an Imperial date is written using only year, it is acceptable to write the year with a backslash, for example; 456/M41. These specify the year on Terra.[4] Only the year is used by Imperial Citizens at large, with most systems using their own calanders for local events,[1] for example the Vigilus Template.

Calculating an Imperial Date

When dates are used for Imperial Administrative purposes, the date is further expanded into four parts; the Check Number (or Source Class), the Year Fraction, the Year, and the Millennium.[1]

0 123 456 M41
Check Year Fraction Year Millennium

Thus a full Imperial Date would be written 0.123.456.M41 or 0123456.M41.[1] To calculate an Imperial date the following method is used:

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. These numbers are based on the Class of the source of the information. Check numbers are vital based on the temporal distortions the warp can cause on real space.[1]

Class Meaning Explanation
0 Earth Standard Date The event took place on Terra.[6]
1 Earth Standard Date The event took place within the Sol System.[4]
2 Direct The source was in direct psychic contact with Terra or the Sol system.[1]
3 Indirect The source was in direct psychic contact with a Class 2 source, but not Terra.[1]
4 Corroborated The source was in direct psychic contact with a Class 3 source.[1]
5 Sub-corroborated The source was in direct psychic contact with a Class 4 source.[1]
6 Non-referenced - 1 Year The source was not in psychic contact with a Class 0-5 source. However it can be linked to a sequence of dates, where one was made in contact with a Class 1-5 source. The maximum period of time seperating this date and the Class 1-5 date is no greater than 1 Year.[1]
7 Non-referenced - 10 Years The source was not in psychic contact with a Class 0-5 source. However it can be linked to a sequence of dates, where one was made in contact with a Class 1-5 source. The maximum period of time seperating this date and the Class 1-5 date is no greater than 10 Years.[1]
8 Non-referenced - Greater than 10 Years The source was not in psychic contact with a Class 0-5 source. However it can be linked to a sequence of dates, where one was made in contact with a Class 1-5 source. The period of time seperating this date and the Class 1-5 date is greater than 10 Years.[1]
9 Approximated Class 9 differs from classes 0-8. It is used when reporting from a source that doesn't use the Imperial Dating System, or when using a historical date approximated from a non-Imperial source.[1]

Year Fraction

For record-keeping, each year is divided into 1000 equal parts, numbered 000-999.[1][Trivia] Assuming Julian years (365.25 days each), this makes each part 8 hours, 45 minutes, 57.6 seconds long.

Year

This is the year within the millennium. Years in the millennium start at 001 and run through to 000[1][4][6] or from 000 to 999.[5][Conflicting Sources]

Millennium

This is the millennium in which the event occurred. These begin at the first millennium AD/CE. They are written down with the letter M, followed by the millenial number.[1] For example the years between 2001 and 3000 would become M3.[5]

Example

Thus an event taking place on Terra at 12:00am January 1st 1998 would be written as 0.000.998.M2.[1] Alternatively the year 1987 could be written simply as 987/M2.[4]

The Chronostrife

Main article: Chronostrife

Between the end of the Great Crusade and the formation of the Great Rift in the ending stages of the 41st Millennium, a conflict had arisen within the Ordo Chronos on the what the correct date was, with the main five competing factional interpretations ranging in date by around a thousand years and some lesser, more heretical tendencies going further. Roboute Guilliman upon learning of this begun to formulate ideas on how to remedy this.[2]

Trivia

TriviaScrollTrans.png

The following information is not to be regarded as officially sanctioned and/ or is not written from an in-universe point of view. A detailed explanation of the term "Trivia" as used in the Lexicanum can be found here.

  • The Anno Domini system used as a base for the Imperial Calendar is never specified. It could be either the Julian or Gregorian calanders. The source refers to "our year 1998" and the "calander we are familiar with", so it is probable that it is referring to the Gregorian calander.
  • An exact method for breaking the year fraction into 1000 equal parts is not given by in-universe sources, however there are some methods avaliable. All give a year fraction as lasting approximately 8.5 Terran hours.
  • DakkaDakka forum user Makr stated in 2009 that the year fraction could be worked out as follows[3] :
    • Figure out what day of the year an event is. (For Example 200)
    • Multiple that by 24. (For Example 200 x 24 = 4800)
    • Add the number of hours an event is to that number. (For Example if the event was at 4pm. 4800 + 16 = 4816)
    • Multiple this by the constant 0.11407955263862231501532129004257 (Giving us 549.407 from our example above)
    • Round this to the closest full number (549). Thus giving you the year fraction.
  • Additional to this Lexicanum user User:307th Artemian noted on 17 February 2015 that Makr's calculation added 24 hours to the number, and thus that you should use the date from 1 day before, then add the hour in the current day.
  • Another Lexicanum user noted on 25 January 2025 that the Constant used by Makr does not account for leap years when converting from the Gregorian calander. 0.11376786 should be used as the constant instead if the year is a leap year.

Conflicting Sources

Main Article: M42

Sources

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