Originally, Gene Roddenberry invented stardates in order to avoid pinning Star Trek down to any specific date in the future (although dialog implied that it took place 200-800 years from now, but in Star Trek II they placed Trek in the 23rd century and the first season of Star Trek: The Next Generation took place in 2364). Stardates were never intended to be examined very closely... but let's look at them closely anyway.

Stardates for the original series run from 1312.4 (Where No Man Has Gone Before) to 5943.7 (All Our Yesterdays). Star Trek: the Motion Picture begins on stardate 7412.6, Star Trek II: The Wrath of Khan begins on stardate 8130.4. From the series we know that 1 stardate equals 1 day. Interestingly enough, if we assume that stardate 0001.00 was 1 Jan 2261 (the centennial of the Federation) counting forward makes stardates actually useful for pinning down events. Using this system, the original voyages of the Enterprise occurred between 2264 and 2279 (true, this is a 15 year period rather than a 5 year period but we can assume the Enterprise under took three separate five year missions). This is not surprising. considering the amount of space she supposedly covered in this time. Under this system, Star Trek: The Motion Picture took place in 2281 (which by the way is completely in keeping with Captain Decker's comment that Admiral Kirk hadn't logged a single star hour in 18 months if the Enterprise returned to Earth in 2279). Star Trek II: The Wrath of Khan took place in 2283 while Star Trek VI: The Undiscovered Country took place in 2287 (see Timeline for more details).

This system works very well until we look at Next Generation Stardates which begin around 41000.00 and assume 1000 stardates equal 1 year. Classic Trek stardates couldn't be 1000 stardates/year because if they were the time between 2266 and 2364 would cover 98000 stardates (!) Clearly at some point Starfleet changed the system. Counting it out to the Federation's bicentennial on stardate 36525.00 the change to a 1000 stardates= 1 year system almost works, in order to bring the stardates into line in time for the start of Star Trek: The Next Generation on stardate 41153.7 in 2364, the stardates for 2361 must run from 38000.00 to 38999.99, so if we assume a Julian/Gregorian style calendar reset where stardates 36525.01 to 37999.99 simply never occur and the calendar is arbitrarily reset to 38000.00, viola! The stardate system remains actually useful for determining exact dates of specific events without having to re-write the Trek universe and without copping out with vague mumbo-jumbo explanations.

A Better Answer
An alternative explanation is that in the original series, stardates referred only to ship board time. That stardate 1250.00 was one year and thirteen weeks since the ship began it's current mission. This makes a modicum of sense. On Earth, we have several calendars in use (Gregorian, Hebrew, Islamic, and Chinese just to name a few). Add a few dozen alien cultures to the mix and it quickly becomes clear that using a Terran calendar aboard a starship is a little problematic. In order to avoid diplomatic incidents, the Federation Council and Starfleet settled on a "stardate" system that would measure ship's time independent of any calendar. Only by comparing a ship's location and time out of port to a standard calendar could stardates be translated into actual dates. This alternative explanation has the added advantage of squaring with Gene Roddenberry's off-the-cuff speculation on stardates as well.

Using the Enterprise as an example, she completed her historic deep space voyage 5.9 years after leaving Earth (2266-2271). Assuming that Captain Decker's refit of the Enterprise is considered to be part of the same mission that we saw in the series, Star Trek: The Motion Picture takes place in 2273, some 18 months after the last Star Trek TV episode (which is borne out from dialogue from the movie) on stardate 7412.6.

Star Trek: The Wrath of Khan eight years later in 2281, remember the "clock" is reset after the Enterprise returns from her "shakedown cruise" documented in Star Trek: The Motion Picture (June 2281 to be precise) and her subsequent reassignment to Starfleet Academy as a Training Ship. Star Trek: The Undiscovered Country takes place over nine years later in 2291 following the reassignment of the Enterprise to line duty.

Its not 100% in line with the official chronology but its close enough for government work.

This is all well and good, but we know from the Star Trek Writer's Bible that in the Next Generation setting, 1000 stardates equal 1 Terrestrial year. It's obvious that this wasn't the case during the series and movie eras because if it were, then Star Trek: The Next Generation would have had to begin around stardate 98000(!) rather than around 41000 as we learned from "Encounter at Farpoint". It is therefore obvious that at some point Starfleet changed the system into a real calendar.

The fact that the Federation would have been politically powerful enough to impose a new dating system on its member states would indicate a quatum advance in the power of the central government between the Classic series and the Next Generation. This is also in keeping with the canon. In Kirk's time, the Federation was more presented as more of an advisory body to planetary governments. Think of the Federation as the NATO and it's members as the NATO nations of today, with Starfleet as the Allied forces. By Picard's time, the relationship between the Federation and the member states was more akin to the relationship between the United States government and the several states with Starfleet acting more like the US Navy than a NATO force.

So then, assuming that the system changed, all we have to do is count back 41 years from 2364. Thus we can surmise that in 2323, Star Fleet converted stardates into an actual calendar, where 1000 stardates equal a year. Again, no need to rewrite the Trek canon and no need to insult our intelligence with mumbo-jumbo cop-outs.

Of the two, I prefer this formula as it is closer to the "official" timeline and is easier to figure out.

Michael Okuda, eat your heart out!