Originally created in order to indicate that Star Trek was set in some undefined future era without actually having to say exactly when the series took place, stardates were little more than largely-random numbers at first. By the time Star Trek: The Next Generation aired, decades later, a more logical, consistent formula was created for stardates: 1000 “star-days” passed per year, which was indicated by the first two digits of the now-five-digit stardate (the better to indicate that nearly a century had passed since the original Star Trek series).
This naturally, introduced as many problems as it solved. The series being set in the twenty-fourth century (as Star Trek II: The Wrath of Khan had established, five years earlier, that the original series was set in the twenty-third century), series creator Gene Roddenberry decided, admittedly arbitrarily, that the first digit of the stardate would be 4; likewise, as it would be the new show’s first seas, the second digit would be 1. Hence, the series was set in the 41st year of this stardate system… yet nearly a century had passed since the original series. Clearly, this must be a new calendar system established just over four decades prior to the new series’ first episode.
Additionally, the final episode of the first season of Star Trek: The Next Generation established, at last, a definitive time frame for the series. The year was clearly stated to be 2364, for the benefit of the episode’s 20th-century refugees (via suspended animation), although no day or month was established to further clarify the matter. The prevailing assumption, which continues to this day in the Star Trek novels, is that stardate x000.0 of each year coincides with January 1st on the Earth calendar. Unfortunately, on-screen evidence in the seventh season of Star Trek: Voyager has proven that not to be the case.
The episode “Homestead” is unique in that it gives both a stardate and a reference to the Earth date. The episode, explicitly set on the 314th anniversary of First Contact Day: the first encounter between Humans and Vulcans, which was seen in the film Star Trek: First Contact and was explicitly stated to have occurred on 5 April 2063. Clearly, the on-screen evidence establishes that Stardate 54868.6 and 5 April 2377 are one and the same date.
Given that there are 1000 stardates per year, and that all evidence has shown a star-year to be equivalent to an Earth year in length, we are left with a ratio of 2.739726027 stardates per Earth day (or 2.732240437:1 for leap years). If x868.6 coincides with April 5th, then it follows that x000.0 would fall 47.961 days later, on 22 May of each year, though the exact time is uncertain. With this information, it is possible to calculate a conversion between Earth calendar dates and TNG-era stardates with a significant degree of accuracy. For example, “Encounter at Farpoint,” which occurred on Stardate 41153.7, was on 17 July 2363; “Emissary,” on Stardate 46379.1, was on 7 October 2368; and “Caretaker,” on Stardate 48315.6, was on 13 September 2370.
The most recent Star Trek film, set in the ST11 alternate reality, introduced yet another stardate system. This system was firmly rooted in the Earth calendar, with the digits preceding the decimal point representing the year, and those following it the date (in a range of 1-365/366). Even within the film itself, this raised several interesting questions. When Captain Robau stated that the stardate was 2233.04, did he mean it was 4 January (day 004) or 9 February (day 040)? This was also inconsistent with Kirk’s stardate reference later in the film of 2258.42 (11 February, day 042). Further adding to the confusion, when Spock asked the Jellyfish computer to state its manufacturing origin, it gave the stardate as 2387 — from that reality’s stardate system, rather than from the TNG-era system (e.g., 64xxx.x), which one would have expected. Could this, then, indicate that the universe encountered in the film was entirely separate from the so-called “Prime” universe, with only minor divergences in things such as the stardate system and nuances of technology to differentiate itself from the “Prime” reality?