From the "Stardates in Star Trek FAQ
Before the Federation was founded, everyone involved in space travel used their own time system. Terrans used the Gregorian calendar and UTC; Vulcans used their own calendar. Initially the Federation used the Terran calendar, just as it used the Terran language and had its headquarters on Earth. This system proved to be extremely unpopular, especially with the Vulcans, who liked a calendar to have some logic about it. (Alternating 30-day months with 31 is fine, but sticking a 28 in the middle of that lot is just silly.)
A Stardate is a fictional unit of measure applied to the passage of time. Gene Roddenberry's Star Trek series (Original airdate- 1966) originated the measure, according to Roddenberry, to place the series well into the future without a strong reference point the viewers could grasp.
While the "Stardates in Star Trek FAQ" (available at http://www.cs.umanitoba.ca/~djc/startrek/stardates/) goes into what some may consider an extremist dissection of the purpose, origin, and calculation of a Stardate, from both the real-world and Trek-world views, I prefer a simpler view.
The Stardates of the original (Roddenberry) series seemed to be somewhat arbitrarily chosen, to a degree. The dates ranged from 1512 to 5928. While some episodes had a wide variance, according to "Star Trek Chronology", one Stardate appears to have corresponded to one day. However, in some episodes, stardates even jumped backwards within the episode. The only explanation offered for this is Roddenberry's relativistic approach stating that passage of time was altered according to the vector of the ship.
"Star Trek: The Next Generation" (Original airdate 1987) brought with it a new Stardate system. They employed a five number system in the form of 4XXXX to distance themselves from the time of the original series. The producers seem to have arbitrarily chosen the first number of the five number system (4), but the second number represents the season (1,2,3...). Interestingly, according to an article at About.com, this system seems to have followed in "Star Trek: Voyager", where the 6th season (would've been TNG's 13th season) Stardates are 53XXX.
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