Each Major Race uses their own dating system, rendering date conversion between systems necessary on occasion. The following article outlines the workings of each system and defines the reference date that determines year 0 in each and how to convert dates between various systems. See History for a list of events.
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The Third Imperium uses a regularized version of the calendar originally developed on Terra. It shares seconds, minutes, hours, days and weeks with the Solomani version, but does not organize them into intermediate months. The Imperial year is exactly 365 days, or 52 weeks with one extra "Holiday" date. The calendar is, naturally, tied to the declaration of the founding of the Imperium by Cleon I.
The Solomani have kept the same dating system that was in use before the Solomani invention of jump drive and coming into contact with the Ziru Sirka. Its reference date is tied to the birth of a popular religious figure from early Terran history.
The Imperial Calendar is heavily influenced by the Solomani dating system, and borrows most of its basic timekeeping units. However, instead of numbering days within a year, as in Imperial practice, days are numbered inside a month, twelve roughly equal subdivisions of the year. Further, the length of the year is subtly different as Solomani calendar still periodically adjusts to synchronize with the sidereal period of Terra around Sol (365.2425 days, thus the Solomani year is 1.0006644 Imperial years). Because of this difference, the first day of the two calendars are 0.74 years apart as of 1116.
The Vilani also count from the founding of the Empire Ziru Sirka. Each Vilani year is ~1.312 Solomani year.
The Zhodani chten is ~0.754 standard years. The calendar uses Olympiads, which is three chten, with an extra day to cover the leap years. An Olympiad is ~2.262 standard years. The Zhodani calendar uses the count of olympiad from the founding of the Psionic Games, plus a marker of which year (1, 2, or 3) into the current olympiad (e.g. 1244.1).
Aslan dating begins from the formation of the Tlaukhu. Each Aslan year is ~0.877 Solomani years.
The K'Kree dating system is also preflight. Each K'Kree year is ~1.2 Solomani, Their calendar starts with the founding of Rak'keer!, their capital city in 8145 Imperial.
Vargr have no accurate calendar, so dating is meaningless, however a Vargr "year" is ~1.03 Solomani years.
The Droyne calendar likely should start around 75,000 Imperial, however the Droyne don't keep good records either.
Hive Federation dates begin with the official proclaiming of Glea as the Federation capital. This occurred in 410 imperial, though the move was not officially completed until 490. While all official dates refer to this calendar, most Hiver nests use local dating systems.
A Cycle (day) is a little over 22 hours. A year is 143 cycles, each cycle simply numbered 1 to 143 with no month or week divisions. Thus, a Hiver year is about half an Imperial one (0.49 specifically).
1105 is 1389 HF; 1111 is 1401 HF (from CT AM 7 HIvers).
Solomani  Imperial  Vilani  Zhodani  Aslan  K'Kree  Event 

3,659 BC  8,145  3,148 PI  639.1  5,442  1  Start of K'Kree calendar 
2,213 BC  6,731  2,046 PI  1.1  5,301  1,186  Start of Zhodani calendar 
AD 1  4,520  361 PI  981.2  2,779  3027  Solomani calendar begins 
AD 473  4,045  1 VI  1,192.2  2,237  3,422  Start of Vilani calendar 
AD 2435  2,083  1,496 VI  2,058.3  0  5,055  Aslan calendar Begins 
AD 4518  0  3,084  2,979.3  2,376  6,788  Start of Imperial calendar 
Read down for date of origin, then across to calendar to convert.
Source (x)  Solomani  Imperial  Vilani  Zhodani  Aslan  K'Kree  Hiver 

Solomani  n/a  (x  4518) / 1.0006644  
Imperial  (x * 1.0006644) + 4518  n/a  (x + 4045)/1.312  (x + 6731)/2.26  (x + 2083) / 0.877  (x + 8145) / 1.2  (x  410)*2 
Vilani  (x * 1.312)  4045  n/a  
Zhodani  (x * 2.26)  6731  n/a  
Aslan  (x * 0.877) 2083  n/a  
K'Kree  (x * 1.2)  8145  n/a  
Hiver  (x/2) + 410  n/a 
Note that converting from a calendar that does not have a zero year (e.g. Solomani, Vilani, Zhodani, K'kree) i.e. the 0 point is an instant in time, rather than a complete year, must account for the "missing" year when going from "negative" dates to positive ones.
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