HOW DO STARDATES WORK?
The Lowdown:
Stardates weren't really meant to be converted into Gregorian calendar equivalents...the exact timeframe of Trek, as a matter of fact, wasn't established until TNG's "The Neutral Zone," which put the current year of that episode at 2364 (since then, that's been used as the reference point for figuring out what "year" it is in Trek).
The Original Series' stardates ran from 1512 ("The Corbomite Maneuver") to 5928 ("Turnabout Intruder"). In the "Star Trek Chronology," the Okudas point out that one stardate unit seemed to correspond to one day, with roughly 57 stardate units elapsing between episode.
Anyway, with TNG, the five-digit SD system was established to distance the TNG era from TOS. "4" was chosen as the first digit, and the second digit stood for the current season of TNG. So first season episodes were all Stardate 41XXX, season 2 eps were SD 42XXX, etcetera. That system continues to this day...it would've been TNG's 13th season, and Voyager's 6th season stardates are in the 53XXX range (2376 by our reckoning). Note that this doesn't jive with the TOS system at all, and the TOS movies' stardates needed lots of jiggering...refer to the Chronology to see how that was done.
Because of the TNG system, it's been generally accepted that one year=1000 stardate units. But then again, writers still tend to follow a 1 unit=1 day convention within the episode, so go figure.
Some fans have adopted a way of converting the current date into a stardate-like format: YYMM.DD, with YY standing for the last two digits of the year, and so on (today would be 9910.08). It's gotten so popular that Paramount has used it for its ST movie ads. But remember that this has nothing to do with the stardate system that the writers use.