Standard Time Units

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.

Anime

 * 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.
 * During battles in Crest of the Stars and its sequels, all elapsed times and expected times are given in seconds, no matter what side the group in question might be a part of.

Literature

 * 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 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
 * 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.
 * The Ekumen of Ursula K. Le Guin's 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", as 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 the standard Gregorian Calendar as their method of timekeeping despite it having 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....
 * 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.
 * In Podkayne of Mars by Robert A. Heinlein, the title character gives the ages of her brother and herself in Martian terms (he's 6 Martian years old -- 11 in Earth years -- and she's a bit over 8 -- 15 on Earth). At one point she complains about the differences in length between Martian days and those on the Earth ship she travels on, and later, on arriving at Venus, makes a joke about suddenly being in her 20s -- middle age on Mars.

Live Action TV
"Green Leader: Drazi Cycle not Drazi week. Drazi Cycle is Drazi year."
 * Star Trek, through Stardates.
 * Farscape does it, though the units are a bit weird since the Translator Microbes don't do conversions.
 * The eponymous station in Babylon 5 uses 24 hour Earth days and other units, as does the rest of Earth Force. Other races presumably have their own sets of units; there is a reference to a 'Narn lightyear' at one point.
 * Specifically, as Babylon 5 is run by the military, the station keeps Earth Standard Time, which is basically Geneva time. One of the main Mars settlements is located in a different time zone, and calls from there to B5 can result in interrupted sleep for the recipient.
 * Also:


 * The 2000s Battlestar Galactica uses standard Earth units and 24-hour military time without explanation. The series takes place, so the best possible explanation is that "standard Colonial time" just happens to exactly resemble Earth chronological conventions.

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.
 * But isn't it a bit odd that they still use the Christian numbering of years? You'd think they'd date it from the Emperor uniting Earth or something...
 * Say it with me now: The God Emperor.
 * Because Emperor said so?
 * It was vaguely hinted in the fluff that the calendar is one of the very few things that survived the Age of Strife, and thus is kept in reverence to the Dark Age of Technology, the legendary Golden Age of Humanity.
 * An interesting variation is seen in one of the Ciaphas Cain books, with a planet that doesn't rotate on its axis. The inhabitants adopt a planetwide system of sleep and work periods that allow them to be effective at the same time.
 * The Traveller Tabletop RPG setting 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.

Video Games

 * In the Mass Effect series, there are two time standards: The standard time we're familiar with, 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.

Webcomics

 * In Darths and 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

 * 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.
 * 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.

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 Trope 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.
 * And also because the second is now defined based on atomic clocks, so that it doesn't change when tectonic activity alters the day length on Earth.
 * A day on Mars is only a bit longer than that of Earth: 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)