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Standard Time Units

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This is a common Speculative Fiction trope. For reasons that should be obvious, planets have years and days of different lengths; months are entirely optional. But almost every sprawling galactic civilization runs on one set of standard time units, nominal years and days used for record-keeping. In universes where humans are a major colonial power, these units are often close to Earth days and years.

When a traveler visits a planet that does not use standard time, the result is Two of Your Earth Minutes. Occasionally overlaps with Alternative Calendar, though many Alternative Calendars are restricted to a single world. If the units are given different names, they are Microts.


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  • Averted and played straight at the same time in ARIA. Aqua (Mars) has its own calendar to represent that it has a different amount of days per year than Earth, but the series does use both that calendar and the standard Gregorian calendar.


  • In Isaac Asimov's later Foundation novels, this is used as a plot point to deduce the identity of Earth, the forgotten homeworld. The standard year and day correspond to no day or year cycle on any known world, but just might correspond to the original.
  • Dan Simmons' Hyperion Cantos novels.
    • Which is both better and worse than most examples. Travel is usually instantaneous so people sometimes go from noon to midnight with a footstep but it's pretty clear that everything everyone knows about anything comes from the World Web, which is standardized.
    • When one of the characters ends up travelling to Earth, which was not destroyed but moved, he expresses surprise that the day exactly corresponds to a 'standard' day in the WorldWeb, before realising that the standard time measurements actually come from Old Earth
  • Vernor Vinge's novel A Deepness in the Sky consciously averts this trope: The major space-faring civilization measures time only in seconds and units derivable from them via metric prefixes (so kiloseconds, megaseconds, etc.). On the other hand, it eventually becomes clear that their computers all count time starting from the Unix epoch.
    • Though they themselves have forgotten the fact, and falsely believe that their clocks start from the time of the first Moon landing. (It's close enough for government work, anyway; the Unix epoch is coincidentally the first Gregorian new year after the moon landing.)
  • The Ekumen of Ursula K. Le Guin's Hainish novels has a nominal standard year for recordkeeping, but due to the difficulty of interstellar travel most worlds use idiosyncratic calendars based on the local year.
  • Most nations in the Honor Harrington series have their own local calendars, but use Earth's years (which they call "T years", like in Terra) for the recordkeeping and general communications. It helps that Earth is still around, and a capital of the largest single polity in the inhabited universe, thus conveniently avoiding the Insignificant Little Blue Planet trope.
    • The only exception would be Protectorate of Grayson, which still uses standard Gregorian Calendar as their method of timekeeping, despite it having a little resemblance to the local orbital parameters, so they had to invent an imaginative way to cope with this problem. But then, their stubbornness is legendary.
  • The drow of Menzoberranzen in the R.A. Salvatore Forgotten Realms novels still use hours, days, and years based on the sun for some reason, even going so far as to enchant a giant stone pillar to serve as an infrared sun surrogate. They don't have any contact with surface-going nations or any reason it would be necessary to synchronize with them.
    • Easily resolved by combining sourcebooks with math. In this setting the drow were banished underground only 10,000 years ago, and drow live for several hundred years. They could have been seeing normal days and nights as few as twenty generations ago.
  • This is a plot point in the Star Wars Expanded Universe novel The Krytos Trap where Corran Horn notes that he's either on a planet so backwater that all local clocks are set to GST, regardless of local time, or... he's actually been on Coruscant the whole duration of his imprisonment.
  • The lack of this is a serious problem on Gor, because it means that pinning down how long ago something happened is almost impossible. Most cities measure time by "when so-and-so was administrator", the desert people have proper calendars but those have years with differing lengths, only Ar counts time in a way that Tarl finds meaningful.
  • When Nameless Technician in Nuklear Age points out that the alien spaceship spotted was exactly one mile in diameter, Dr. Genius disbelievingly points out what a coincidence that would have to be. Of course, there is a good reason.
  • Utterly and completely averted in many Greg Egan novels, which are frequently told from the point of view of conscious AI entities running on weird and whacky computational frameworks. (The weirdnessess are often plot points, and the changing ratio of perceived time versus objective time as the framework changes are also plot points.) There is at least one book (Diaspora?) where you have all that going on AND relativity in the mix. "Rushing," where the character runs their clock incredibly slowly to wait for the Universe to do interesting things in a short space of perceived time, is often used to make the cosmology useful to the plot.
  • Redshirts has universal time used on all Universal Union ships and bases, but it seems most planets keep some sort of local time since Dahl has to work out the time in Boston when calling an old friend from a distant space station.

     Live Action TV  

  • The eponymous station in Babylon 5 uses 24 hour Earth days (measured in "Earth Standard Time", apparently Geneva time) and other units, as does the rest of EarthForce. Other races have their own sets of units, although most of them normally use human equivalent names when discussing time due to the nature of the station as being run by EarthForce.
  • Battlestar Galactica (2003) uses standard Earth units and 24-hour military time without explanation. The series takes place in the Neanderthal era, in a society that has no knowledge of the planet Earth, so the best possible explanation is that "standard Colonial time" just happens to exactly resemble Earth chronological conventions.
  • Farscape does it, though the units are a bit weird since the Translator Microbes don't do conversions.
  • Star Trek, through Stardates.

     Tabletop Games  

  • The Imperium of Warhammer 40,000 uses Earth years in a continuation of the Gregorian calendar, with the year nominally divided into 1,000 parts for record-keeping purposes. They do use a unique notation, though. Instead of "38,420 AD" they would write "420 M39," meaning 420th year, 39th millennium. There's also an official way of recording how accurate the date is considered to be (the vagaries of space travel and communication in the setting mean they're lucky of it's within ten years in many cases), but few authors bother with the whole 10 character date.
    • An interesting variation is seen in one of the Ciaphas Cain books, with a planet that is tidally locked, meaning that the planet only rotates once in the time required to orbit the star. Since the ambient sunlight is constant for a given location and doesn't change based on the time of day the inhabitants don't use time zones and instead adopt a planet wide time so that everyone is awake or asleep at the same time worldwide.
  • The Traveller Tabletop RPG The Third Imperium used a 365-day year, probably because of tradition: it was based on the Second Imperium, which had been created by Earth humans conquering the First Imperium.
  • Space 1889 Mostly averted. Martians use their own time units, but since the Martian day is 24 hours and 40 minutes conversion is not really necessary. Martian year is close slightly less two earth years making conversion easy.

     Video Games  

  • In the Mass Effect series, there are two time standards: Earth standard time, used by the Alliancenote , and the Galactic Standard Time, which the rest of the galaxy goes by. In the Galactic Standard Time, a day is divided into 20 hours, each hour is 100 minutes long, and each minute is 100 seconds long. However, 1 galactic second is about twice as fast as 1 Earth second, so it's basically a 50-second minute, and the days would be 15% longer than an Earth day. For convenience's sake, the narrative goes by the Earth Standard Time.


  • In Darths & Droids, it becomes clear that the GM has not set up a standard calendar for the universe when everyone's debating whether Jim can use his Fate Manipulation ability again to avoid his character's death. This despite the Republic in Star Wars having using a standard calendar (based on the calendar of its central planet, Coruscant); of course, Jim's character did have to die in that scene.
  • In Unity, the time units are Metric intervals based on the internal circadian period of the ship. A day is divided into 10 decs, which are in turn divided into 10 kaysecs, which are in turn divided into 10 centis of 100 seconds. On the other side, 5 days become a dec, and 5 decs become a round, as explained in the comments on this comic.

     Web Original  
  • Orion's Arm uses fairly hard science and thus has to accept the problem posed by relativity. As a result no one has any idea what year it is by our calendar (except GAIA) and every planet has not just different length days and years but experiences time at a different rate.
  • Void Dogs is set in deep space, so the issue of different planets having different day lengths hasn't been addressed. It's been hinted that the standard is actually a 28 hour day.

     Real Life  
  • 24-hour days are used in polar regions of the earth, even at times when the sun may not set for months on end in summer, and not rise for months in winter, and thus the idea of "days" is meaningless.
    • Justified in that it still represents the amount of time it takes for the sun to make one circuit in the sky, even if it doesn't set. And, you know, the sleep cycles of the people there.
    • Likewise, the use of the 24-hour clock (set to Zulu Time, i.e. GMT) is maintained on submarines at sea and in space, where the day-night cycle is completely irrelevant.
    • On the other hand, time on space telescopes is allocated in kiloseconds (and megaseconds, for really massive projects), probably because the second (unlike minutes, hours and days) is an SI unit.
  • A day on Mars is only a bit longer: a sidereal day is 24 hours and 37 minutes (vs 23 h 56 m on Earth), and one solar day is 24 h 39 m (vs 24 h on Earth)
  • The second is now defined based on atomic clocks, so that it doesn't change when tectonic activity alters the day length on Earth. It also means you don't have to be watching the Earth to measure a second IN SPACE!