In my novel, I have created an intergalactic empire that predates humanity by millions of years. The only system I am having a difficult time creating, for this empire, is a Calendar system that operates off of cycles of the universe; not of a local planet. However, finding information about the cycles of the universe is like looking for a needle in a haystack.

Does anyone have any information about the cycles of the universe that might be helpful for creating a kind of intergalactic calendar?

In my system there should be an intergalactic date, a planet symbol, and then base a calendar off that planet so that there’s five sets of digits in the dating system.

For example:

  • galactic year - GY (?)
  • Planet - P (which planet)
  • Local Year - LY (planetary year)
  • Local Cycle - LC (planetary month)
  • Local Sub-Cycle - LSC (planetary day)

Time is based on a UTC type of synchronization using what amounts to an atomic clock when in space which is synchronized to a local planetary subdivision of a day using a base of that planet's cycles, such as a base 10, base 12, base 24, base 30, base 100, etc. - a numeral system for pre-colonized worlds such as earth.

  • day (light / dark cycle)
  • hour (large time grouping)
  • minute (medium time grouping)
  • second (small time grouping)
  • millisecond (tiny time grouping)

The empire is highly advanced and logical so I want to ensure the logic applies also to their calendar system.

Any suggestions or recommendations are greatly appreciated.

Edit: This question is about observable phenomena in the universe that could be measured and used for time keeping, it's not a question of how to make a calendar. 200 million years per revolution of the milky way is on the right path. How about cycles of the universe as a whole?

  • $\begingroup$ Welcome to worldbuilding. Please take the tour and visit the help center to better understand our community, its rules and standard. $\endgroup$
    – L.Dutch
    Commented Sep 4, 2019 at 12:25
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    $\begingroup$ The galactic year for our Milky Way galaxy is approximately 200 million years. An intergalactic empire may use the duration of its home galaxy as their base galactic year. Since culture plays a part in decisions like these for example, their standard galactic year might be the average of the all galaxies in their intergalactic empire. You will need to balance natural processes with whatever cultural factors are involved. $\endgroup$
    – a4android
    Commented Sep 4, 2019 at 12:36
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    $\begingroup$ If you strive for logic, I would suggest you reconsider the proposed system. Galactic year makes no sense as it is waaay too long to be practical, moreover might not be known precisely enough. For planetary year and day you need a fixed origin. Planetary month is an arbitrary construct you do not need at all. Generally, the only thing(s) you need is standardised time point and time length, so your time will be somany Units since Origin of Time. $\endgroup$
    – Mori
    Commented Sep 4, 2019 at 13:13
  • $\begingroup$ I suspect that "highly advanced and logical" needs to be more defined, if relevant. I know a lot of very logical folks who weirdly use ancient base-12 timekeeping and base-7 calendars that have days added and subtracted rather haphazardly. They claim doing so keeps them on time to their appointments because everybody else uses that strange old cobbled-together system. Thoroughly silly...but quite logical. $\endgroup$
    – user535733
    Commented Sep 4, 2019 at 13:39
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    $\begingroup$ Presumably on a planetary surface where people still have natural daylight and night and seasons, they will use a local planetary calendar and time system developed by the natives or colonists, and also a standard empire wide calendar and time system. With the wide availability of technology in a galactic empire every time and date can be expressed in one or another of the two systems or both, as the user desires. On artificial space habitats people might possibly just use the empire wide system or possibly both the imperial system and that of the major planet in their solar system. $\endgroup$ Commented Sep 4, 2019 at 18:54

7 Answers 7


Use the frequency of relic radiation to determine time since the Big Bang.

Relic radiation (a cooler term for a fiction than Cosmic Microwave Background, I think)is detectable everywhere in the universe. The frequency of this radiation has gradually been redshifting since its origin shortly after the Big Bang. The observed frequency corresponds to how long it has been since the Big Bang. It does not matter where you are. The frequency of relic radiation could be used as a date.


The cosmic microwave background (CMB, CMBR), in Big Bang cosmology, is electromagnetic radiation as a remnant from an early stage of the universe, also known as "relic radiation". The CMB is faint cosmic background radiation filling all space. It is an important source of data on the early universe because it is the oldest electromagnetic radiation in the universe, dating to the epoch of recombination... The accidental discovery of the CMB in 1964 by American radio astronomers Arno Penzias and Robert Wilson... and earned the discoverers the 1978 Nobel Prize in Physics.

CMB is landmark evidence of the Big Bang origin of the universe. When the universe was young, before the formation of stars and planets, it was denser, much hotter, and filled with a uniform glow from a white-hot fog of hydrogen plasma... The photons that existed at the time of photon decoupling have been propagating ever since, though growing fainter and less energetic, since the expansion of space causes their wavelength to increase over time (and wavelength is inversely proportional to energy according to Planck's relation). This is the source of the alternative term relic radiation...

Assuming the universe keeps expanding and it does not suffer a Big Crunch, a Big Rip, or another similar fate, the cosmic microwave background will continue redshifting until it will no longer be detectable

The amount of redshift present at any given time corresponds to the time since the radiation originated. Time travelers emerging into an unknown epoch could take a reading of relic radiation and use detected frequencies to calculate when they had arrived.


As pointed out rather neatly by @PiggyChu001, time changes relative to the (among other things) gravitational differences near the sources of two clocks. In fact, it changes according to this linear equation:

$ t_a = t_r * \sqrt{1-\frac{2GM}{rc^2}}$

where ta is the actual time, tr is the recorded time (at your position in space), r is the distance from the object causing the gravity, and $ \frac{2G}{c^2} $ is just a constant.

The fun part about this is that it comes from special relativity, and experiments at Harvard show that this is a direct result of special relativity acting on space-time. The upshot of this for you is time is affected only by the magnitude and direction of gravitational forces acting upon it. If you can calculate the strength of a gravitational field within a system, as well as its centre, you can substitute Newton's Law of Universal Gravitation:

$ F_r = \frac{Gm_1m_2}{d^2}$

to find the resultant force and singularity of your local gravitational field (repeat for every mass in your field to get one universal result). You can then normalise the time between any two points in your galactic empire by plugging in the time at one end (ta), calculating the relative difference in gravitational field strength (change $ \frac{2GM}{rc^2} $ to $ \frac{2gr}{c^2} $, where g is your local field strength and r is the distance from the point where this field has a net force of zero) and bob's your uncle. A year at the Galactic capital can be converted into a year on the frontier. Use atomic clocks for best accuracy (caesium is always a reliable choice), and again make sure to account for the difference in local field strengths. In reality, you'll need to take into account gravitational red shift due to the expanding universe, but for world building the above is mathematically justifiable.

Hope it helps in your quest for Galactic domination!


Finding this kind of "Standard Time System" is next to impossible!

Because, like it or not, "time" is actually all "relative"!

"Every" planet has its own speed of time due to the difference of the gravity.

Even if you have come up with some system for the empire people to follow, the time in Planet A WILL be different from the time in Planet B! For example a year in Planet A is actually 13 months in Planet B!

Even though the said difference will be "VERY" small! But it will add up eventually!

Especially if your empire had existed for so long (millions of years), that difference will definitely be quite significant!

Unless you had come up with some kind of device or something to take the Gravitational Time Dilation into account and adjust "the length of unit time", in other words, "the speed of time" accordingly, the time in your empire "WON'T" be synchronized!

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    $\begingroup$ The difference will only add up if you let it. On our planet, the keepers of UTC regularly add leap seconds to the clock to keep it in line with celestial motion. There's no reason why you couldn't make frequent small adjustments to keep clocks on different planets synchronized, while having virtually no impact on anyone's day-to-day life (did you notice that 2015 was 1 second longer than 2014?). This issue is very easily fixed. $\endgroup$ Commented Sep 4, 2019 at 13:42

You are going to have a problem there because a galactic empire by definition spans a space that is hundreds of thousands of light years across. Relativity plays a more important role in measuring time when you consider this, because things far away from you may be moving at relativistic speeds relative to you.

For example, S2 can reach orbital speeds of up to 5,000 km/s (compare with Mercuy's tame 47.6 km/s). That's 1/60th of the speed of light. Someone on a planet orbiting S2 will measure time differently than someone on Earth.

Enter a thing called relativity of simultaneity. If you want the full mathematical elaboration of the phenomenon, follow the link. In very layman terms, if we are:

  • Sufficiently far from one another...
  • And moving relativistically as I described above with Earth and S2...
  • And we observe the same set of things happening anywhere in the universe...
  • And those things are not causally connected...

Then we will agree that those things happened, but not in which order. For example, a supernova happens in Draco while a black hole devours a spaceship in Orion. It may be that, for me, the spaceship was devoured first and the supernova went off later. For you it's the opposite - first the supernova happened, and then the ship fell onto the black hole.

Given all this, very exact and precise timekeeping on a galactic scale is impossible. What you can do is:

  • Assign a galaxy rotation period as a galactic year. The Milky Way completes a full rotation every 200 million years, and everyone everywhere can measure that;
  • Let every planet or star system define its own timekeeping;
  • Broadcast a signal at fixed intervals from the center of the galaxy, to act as an "exchange rate" of time among different systems. Make sure each signal sent references the previous one in some way, to establish a causal relationship. A simple counter increment will do.

When two different systems wish to discuss passage of time, each can compare their own time passing and event order to the cycles of the central broadcast. They can then infer how fast time passes and how order of events differ between them by measuring the comparisons against each other. In such a way, each planet's time will be like a currency in an exchange agency, with the central broadcast acting as the Dollar or Euro of the house.


Easyer and more universal approach: Take the planck-time, the smallest physically possible unit of time, figure out what kind of number system they are using and devope something like the SI-system-prefixes. Name the appropriately long time spans creatively and use them like the nearest equivilant. For example let's meet in one qitta-tp (maybe leave out tp, shorthand for Planck time). A qitta is roughly eauivilant to, say 1,3 days or ca. 32 h. A good unit to replace the day with.

Use the big bang as t = 0. Alternatively you can do this with the aliens main time unit. I once designed such a system with secounds as my base and the new day had about 27 hours.

Such a system is very universal and great for spacecraft and interplanetary affairs. If you megastructure or planet has a rotation and spin time (year and day) that is not so rediculessly short or long that everyone ignores it, it might surplant the galactic system. However for this same reason the physics based system will be superior to the galactic rotation system. A galactic rotation is way too long to be useful and can cause a lot of xanflict because people might measure it differently or fudge with the objects timekeeping is based on to justify some political agenda. Sounds rediculess? Well, the Maja did it.

  • $\begingroup$ The Planck time is not a quantum of time. It is just the smallest time interval which is physically meaningful. As far as we know, time is not quantized; there is nothing stopping a time interval from consisting of 173.2050808 Planck units. There is no conceptual difference whatsoever between using the Planck time as the unit of measurement as compared to using the SI second. $\endgroup$
    – AlexP
    Commented Sep 4, 2019 at 15:13
  • $\begingroup$ But won't everyone get these same time when they attempt to measure the smallest possible quantity? I've heard it would be great for communication with aliens fo that reason. And other, diverged wings of your galaxy spanning civilisation might as well be aliens. $\endgroup$ Commented Sep 4, 2019 at 16:05
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    $\begingroup$ Yes, they would. But it is still the exact same thing as using SI seconds; the SI second also has a fundamentals-based definition, so that everybody can reproduce it. (And, at our level of understanding physics, with veeery much greater precision than measuring the Planck time -- the value of the Planck time is only known with 7 significant digits, whereas the standard definition of the second ensures 10 accurate digits.) $\endgroup$
    – AlexP
    Commented Sep 4, 2019 at 16:31
  • $\begingroup$ @AlexP I would use the second for any humanity-origin civilisation, but is the fundamental based definition of universal enough that aliens could develop independently? $\endgroup$ Commented Sep 4, 2019 at 19:40
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    $\begingroup$ I know. The magic number 9192631770 was chosen so that 24 * 60 * 60 seconds make exactly one mean solar day computed for January 1st, 1900, to ensure historical continuity with the previous definitions. When the SI changes definitions (to make them more precise or more reproducible) they always take very great care to keep the actual values of the units of measurement unchanged, within the best accuracy available. $\endgroup$
    – AlexP
    Commented Sep 5, 2019 at 8:33

Use Local Time when convenient, and Imperial Time when you must

Turns out computers are really good at converting between the two.

Every species in the empire, and the colonist on every planet, will be adapted to different cycles, so why bother forcing everybody to adapt? Use the local time for local events.

This way, Imperial Time can be simple and similar to the way computer time works - the Unix Epoch is measured in seconds since 1970. The computer converts time-in-seconds into whatever year-month-day format the user needs at that particular moment. It doesn't really matter if an Imperial Quarb is a second or a day or a week. My display doesn't show the date to me in Quarbs - it shows me in familiar local Barrfs. Nobody needs to convert in their head.

If my dentist wants me to get half-Barffly checkups, I'll put the appointment on my local Barff calendar and I'll be there.

In the meantime, my space-liner to Rigel is scheduled to arrive at 10385678332 Imperial Quarbs, which my display happily converts into local Barffs/Narffs/Glarffs for me. I'll be there on time, too.

Some locations (spaceports, Imperial offices, courts, etc), are likely to have clocks that simply display time in both formats, but that's mostly for show. Some documents will need to use Quarbs to prevent confusion, but so what? Asking the nearest toaster or lamp-post for the current time in whatever format you like is trivial. Just like asking your PC or Phone for the time in a different format is trivial for us today.


The imperial timescale is calculated from a significant point in the imperial history using the calendar of the imperial capital.

If you have a communication system that works instantaneously over the full width of the empire then the whole thing becomes moot, the capital broadcasts a time signal. If it takes three years to send a message then it becomes meaningless what the datetime is in the capital.

The need for universal time only applies when universal time becomes a necessity. Circular logic yes, but this comes out of railway time. The time the train arrives only matters when it's moving fast enough to matter. If it takes 3 years to get from the capital out to the remote worlds then there's no reason at all to have a matching time calculation, it's barely worth tracking the years. People may know the approximate date in the capital but it's effectively meaningless.

This only matters for imperial edicts, which will bear the imperial timestamp. When the edict arrives at the world in question, the local timestamp will be added and the edict applies from that moment. It can't apply before it's received so the actual moment of issuing is irrelevant to the provincial worlds.


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