The Galactic Encyclopedia - Entry 002
Tracking the passage of time requires two things: a fixed point in the past from which to start counting and one or more units of time to tell how long it has been. Humans invented many calendars during the Human Era based on several different natural phenomena, most commonly the rotation of the Earth around the Sun and around itself.
Based on these same principles, towards the end of the Human Era, as humanity started to become an interstellar species, they developed the Stellar Calendar, which remained the main way to track time throughout the Galaxy right up until the foundation of the Galactic Empire —and even then, the new Imperial Calendar was just an extension of the Stellar Calendar.
At the same time, all colonies, and later the bigger factions, developed local calendars that more closely matched their local conditions, for it is hard to tell time in a unit that doesn't match the solar day. But crucially, at almost all points in our story, there is one agreed-upon global time tracking system (a Stellar Calendar) so that local time and date can always be easily converted to global time and date.
The Stellar Calendar
The original Stellar Calendar is based on three units of measure: standard second, standard day, and standard year.
The standard second is defined as 9, 192, 631, 770 periods of the ground-state hyperfine transitions of cesium-133, which roughly maps to 1/86400 parts of an Earth day. One standard day is thus the average time it took old Earth to spin around its axis (or the time for the Sun to take the same position twice in the sky). One standard year is the time it takes old Earth to complete one orbit around its Sun.
The date format in this calendar is in the form YEAR.DAY:SECONDS
. Thus, when an event is dated as 12,378.56:23056 SD
, that means it happened in the year 12,378, on the 56th day of the year, and on the 23,056th second of that day (approximately 6:20 am on a standard Earthly day).1
Critically, the YEAR numbers were set to begin roughly 12,300 years before the beginning of the Exodus Era, or equivalently, around 10,000 years before the beginning of the ancient Catholic Calendar.2
During the Exodus Era, all ships were synchronized to this Stellar Calendar to keep track of time in synch with each other. Even though they might never revisit Earth, when two different colonies would meet on a future date, they could align their historical records to a shared calendar.3
Local Calendars
Upon discovery and arrival at a habitable planet, the ships’ AIs were instructed to compute the local durations of years and days and thus define a Local Calendar with the exact mechanism as the Stellar Calendar but centered around that planet.
The moment of landing would become the year zero, day 1, and would be recorded in the ship’s logs. From then on, all events recorded during the colony's establishment could be tracked in stellar and local times. Dates in a Local Calendar are formatted in the same way as the Stellar Calendar but referencing the local name, such as 7.67.8540 Eos LD.
4
Since each planet could have a completely different day and year duration, it is not so simple —for a human— to convert back and forth between local and stellar dates. However, the conversion is mathematically well-defined and easy to perform with a simple computing device.
Thus, even though different colonies would have different local calendars, by converting back and forth to stellar dates, they could easily sync up their local historical records.
During the Expansion Era, as colonies became more and more interconnected, galactic communication, as well as interstellar travelers, fell back to using the Stellar Calendar, while locals would use their planetary counterparts in everyday life.
Imperial Calendar
The other major calendar used throughout the Galaxy was the Imperial Calendar, again based on the same time-tracking mechanism but using the founding date of the Galactic Empire as the year zero and the orbital parameters of Imperial City as the frame of reference for the duration of a year and day.
Though the Empire tried hard to force this new calendar, it took a long time for everyone, especially those on the Galaxy's fringes, to switch from the old Stellar Calendar completely. Fortunately, conversion back and forth is as simple as with any local calendar.
Thus, dates in Imperial Calendar have the same format as Stellar dates but are offset around 18,400 years and end in ID.5
Non-standard calendars
The previous calendars are collectively called “standard” calendars because they are based on the same standard principles for time tracking. At its height, more than 95% of the Galaxy’s population used these calendars.
However, less civilized or otherwise isolated populations also developed their own time-tracking mechanisms. Native populations in isolated planets would develop systems similar to those used in old Earth, such as dividing the solar years into seasons based on weather patterns or the influence of nearby celestial bodies.
This phenomenon was more widespread during the Genesis and early Expanions Eras, but much less in the Imperial Era. However, during the Great Scattering, as a large part of the scientific knowledge was lost for a majority of the Galaxy’s population, many returned to these more primitive systems for time-tracking.
SD means Stellar Date, or alternatively, Standard Date.
Imperial historians believe this was motivated by the idea that the first human settlements had appeared around 10,000 years before the beginning of the Catholic Calendar. Thus, such a reference point was a more global, culturally agnostic point of origin for human history.
Due to relativistic effects, the relative time in each ship's frame of reference could be significantly different. However, the ships’ AIs would account for this effect once landed and recompute all internal logs to match the Stellar Calendar in a frame of reference centered in the Galaxy’s center of mass and in relative repose with respect to the Galaxy as a whole.
LD means Local Date.
ID originally meant Imperial Dominion but was commonly called Imperial Date by almost everyone outside formal Imperial documents.