Stardates were in use as far back as the 2150s, although Earth had not yet adopted the system. Among the species who did utilize stardates at the time were the Xindi. In 2154, Degra, a Xindi-Primate, sent a coded message to Enterprise containing a stardate for when Enterprise should rendezvous with Degra's ship at specified coordinates, which were also contained within the message. Vulcans also seemed to have an understanding for stardates at that time, as T'Pol was able to calculate that the given stardate was three days away. (ENT: "Damage")
In the 24th century, approximately one thousand stardate units pass for every Earth year, although there are variations in the system at some points in the timeline. (Star Trek: The Next Generation; Star Trek: Deep Space Nine; Star Trek: Voyager)
In an alternate timeline, the combat date replaced the stardate as the dating system used by Starfleet during their war with the Klingon Empire. This was indicative of the militaristic nature of Starfleet in this timeline. (TNG: "Yesterday's Enterprise")
According to Gene Roddenberry, stardates were created to avoid placing the events of the original series within a specific year. At present, no canonical information exists regarding how the stardate system works and exactly why it is needed, although certain regularities can be observed in some of its incarnations. According to Roddenberry in The Making of Star Trek, stardates adjust for shifts in relative time which occur due to the vessel's speed and space warp capability. Stardates specified, in a log entry for example, must be computed against the speed of the vessel, the space warp factor, and its position within our galaxy in order to give a meaningful reading when read back.
Stardates often pass with little rhyme or reason, since they are simply manufactured by the show's creative personnel and not intended to be analyzed for any type of pattern. Gene Roddenberry and many other members of the production staff maintained that perceived irregularities in stardates stem from our own lack of understanding of subspace physics and space warp factors.
In the original Star Trek, stardates were usually four digits long, followed by a single decimal place. To show the passage of time, the producers of Star Trek: The Next Generation decided to use five-digit stardates. They chose to begin the stardate with the number 4, chosen because the program was set during the 24th century. The next digit signified the season of TNG (so TNG Season 1 episodes had stardates 41xxx.x, Season 2 had stardates 42xxx.x, and so forth). The remaining digits increased gradually over the course of the season, from xx000.0 to xx999.9. Under this system, 1,000 stardate "units" were approximately equal to one year, since that is the normal timespan between two TV seasons. The writers of the Star Trek Chronology further simplified the system by having a Gregorian year start at 000 and end at 999, although this does not fit all references in the show, such as a Diwali celebration around stardate 44390, too early in the year according to the simplified system. (TNG: "Data's Day") Stardate 41986.0 occurred during the year 2364 which would mean the origin of the 24th century stardate system was around the year 2323, assuming it had been reset to zero and not integrated with previous stardates in a more complex manner. (TNG: "The Neutral Zone")
Star Trek: Deep Space Nine and Star Trek: Voyager continued the TNG format, so that DS9's stardates ranged from 46379.1 to 52861.3, and Voyager's stardates ranged from 48315.6 to 54973.4. Star Trek Nemesis, the latest Star Trek story in the 24th century, had a stardate of 56844.9, showing that it took place fifteen years after the first season of TNG. The USS Brattain was commissioned on stardate 22519.5, nineteen years before the first season assuming a regular increase, making it the earliest mentioned stardate in the 24th century. (TNG: "Night Terrors")
In addition, it has been suggested by some sources that the final digit of a stardate (following the decimal place) represents one-tenth of a 24-hour day – for example the TNG Bible claims that "the digit following the decimal point may be regarded as a day counter". It is not clear how this can be reconciled with the notion that 1,000 stardate units are equal to one year. If 1,000 stardates equal one year, then one stardate unit should represent approximately 8 hours and 46 minutes. In TNG: "Time Squared" it was indicated that stardate 42679.5 was six hours in front of stardate 42679.2, indicating that 0.1 stardate unit was equal to two hours.
Although the vast majority of stardates are given with only one digit following the decimal point, the captain's log in TNG: "Code of Honor" is recorded with two digits (41235.25 and 41235.32) and other references have two, three or even four digits, as in TNG: "The Child", where a stardate of 42073.1435 is seen on a viewscreen in the Observation Lounge or in VOY: "Relativity", when Seven of Nine travels back in time from 52861.274 to 49123.5621. Occasionally there are no digits, such as when "today's date" is given as stardate 47988. (TNG: "All Good Things...")
Template:2009spoilersection According to writer Roberto Orci, it was intended that stardates in the alternate reality be composed of the calendar year and the day of the year (from .1 to .365).   For example, 2258.42 is the 42nd day or February 11 of the year 2258, although it is unclear how Captain Robau's "twenty-two thirty-three oh-four" fits into the system.
Stardate 2258.42 is stated shortly after the destruction of Vulcan twenty-five years later, supporting the rate of a unit per year. Finally, Spock's Jellyfish gives its commissioning stardate as 2387, consistent with Spock's arrival from 129 years in the future and the events of the Star Trek: Countdown comics.
There have been many inconsistencies with stardates throughout all of the Star Trek incarnations. Examples include:
- In TOS: "Where No Man Has Gone Before", the stardates within the episode progress by 1.4, from 1312.4 to 1313.8, in what could not be more than a few days, yet the birth stardates of Gary Mitchell, Elizabeth Dehner and James T. Kirk are given as 1087.7 (onscreen dossier age: 23), 1089.5 (onscreen dossier age: 21) and 1277.1, respectively. Kirk's birth is only 35.3 units before his first captain's log, about one for every year of his life, while the other two dates apparently use this rate as well, even if their values are much lower than Kirk's stardate of birth.
- In TOS: "The Corbomite Maneuver", the first stardate given is 1512.2 and the second one, given eighteen hours later is 1513.8, suggesting that 1.6 stardates equals eighteen hours or one stardate equals eleven hours and fifteen minutes.
- In TNG: "Skin of Evil" where Tasha Yar is killed, the stardate of 41601.3 is given. In the previous TNG: "The Arsenal of Freedom", a stardate of 41798.2 is given, and Yar is still alive and well, most likely due to a production error related to the fact that those two episodes were filmed out of order.
- In TNG: "Datalore", Riker dropped one of the numbers in his log, stating "Stardate 4124.5".
- In DS9: "Second Sight", stardate 47329.4, it is said to be the fourth anniversary of the Battle of Wolf 359, stardate 44002.3.
- In DS9: "In Purgatory's Shadow", Captain Sisko gives mention to the "recent Borg attack" depicted in Star Trek: First Contact. However, in the very next episode, DS9: "By Inferno's Light", the stardate of 50564.2 is given. In First Contact, the stardate of 50893.5 is given, suggesting that it takes place months after "In Purgatory's Shadow".
- In VOY: "Unimatrix Zero, Part II", set during stardate 54014.4, Tuvok mentions that his date of birth is stardate 38774. Assuming the generally accepted rule that 1,000 units equals one year, this would mean that he was fifteen years old at the time of the episode.
- Stardate at Memory Beta, the wiki for licensed Star Trek works
- The Stardate FAQ - primarily develops one particular theory of stardates that has gained some currency
- Determining Calendar Dates from Stardates - has calculations and calculators based upon information from the television series' and movies
- Star Trek logs - database of stardates and logs from the Star Trek films and television series