The stardate is a comprehensive calendar system and unit of time measurement in the United Federation of Planets. The system of measuring time was introduced as a singular way for the many different civilizations of the Federation to have a unified time system.
The Federation were not the first to develop a stardate system. In the 2150s, the Xindi used them as a way of recording time. The system was also considered by the Vulcans at the same time. (ENT episode: "Damage")
There are multiple systems of rendering stardates for easy understanding, leading to there being different expressions of stardates that could possibly refer to the same date. For example, the reference stardate system includes a prefix to show the progression of centuries, while shipboard stardates omit such prefix arrangements and are rendered differently. (FASA RPG module: Cadet's Orientation Sourcebook, et al. FASA RPG modules and supplements)
During the USS Enterprise (NCC-1701)'s first five-year mission between 2246 and 2251, the Federation were still preferring to use Terran dates as the standard time-keeping preset, but Captain Robert April decided that a better system needed to be used and outlined to Starfleet Command, upon the Enterprise's return to Earth in 2251, how the stardate system was more precise as a spacefaring method of timekeeping. (TOS - Enterprise Logs short story: "Though Hell Should Bar the Way")
- There have been a number of changes made to the way stardates are calculated and used in Trek over the history of the franchise. While the system in the Original Series had no clear pattern to the progression of stardates, in the latter series set in the 24th century, it was generally accepted that 1000 stardate units equaled one solar year, although some sources occasionally contradicted that simplification. By the alternate reality of the new 2009 Star Trek, stardates had been simplified to a decimal following the four-digit year.
- Early in the year 2233, as expressed by Captain Richard Robau of the USS Kelvin. (TOS movie & novelization: Star Trek - The stardates in the film and new franchise materials consisted of the Terran calendar year followed by a decimal. These could be considered an alternate expression compared to concurrent stardates that do not use the year as a prefix.)
- Early in the year 2233, as expressed by Lieutenant Commander Gary Mitchell of the USS Enterprise representing Captain James T. Kirk's birthdate. (TOS episode: "Where No Man Has Gone Before" - The stardates in TOS were non-sequential and could be considered an alternate expression compared to concurrent stardates that use the year as a prefix or use the reference stardate prefix to establish sequence.)
- The early 2240s, as expressed by the Enterprise library computer referencing events on Tarsus IV. (TOS episode: "The Conscience of the King" - Non-sequential TOS stardate.)
- The early 2250s, as expressed by Captain Christopher Pike of the USS Enterprise. (EV comic: "Flesh of My Flesh" - The occurrence of this stardate is not specified, but it probably represents a non-sequential TOS stardate.)
- The mid-2250s, as expressed by Number One of the USS Enterprise. (EV comic: "The Flat, Gold Forever" - The occurrence of this stardate is not specified, but it probably represents a non-sequential TOS stardate.)
- Late in the year 2364, as expressed by Lieutenant commander Data of the USS Enterprise-D. (TNG episode: "The Neutral Zone" - TNG, DS9 and VOY used a system of sequential stardates with 1000 units equaling one solar year)