Warning: Geek stuff beyond this point.
My stardate calculator is based upon the Stardates in Star Trek FAQ by Andrew Main. I have, however, made a couple of different assumptions. First, and most important, is that I assumed that Stardates increased from 0 at a certain point and that they do not reset at any point. The second is that the timebase works in terms of seconds, as the second is the base unit for measuring time.
In Andrew Main's FAQ, he determined that the Episode “Charlie X” took place in November of 2266 because of it's mentioning of the Thanksgiving Holiday (which featured a voice cameo by the Great Bird of the Galaxy himself) and made the final episode stardate-wise about 2 1/2 years afterward. This gave him a rate of about 5 stardate units (sdu) per day. A rate of 18,000 seconds per stardate unit comes out to 4.8 sdu/day, which Mr. Main also found acceptable.
For the second through sixth feature films, Mr. Main compares the dates of the 3rd and 6th movies and determined that the stardates increase at about 0.5 sdu/day, or 10 times slower that during the original series. Consequently, I used a rate of 180,000 s/sdu (0.48 sdu/day).
For the first movie, Mr. Main required the stardate range to slow considerably from both the series and the rest of the movies. Mr. Main figured a rate of 0.15 sdu/day to be optimal; however, he finds 0.1 sdu/day to be more workable. A rate of 600,000 s/sdu works into the range, with a rate of 0.144 sdu/day. I subsequently calculated the points of intersection to be at SD 7320 (11 March 2270) and SD 8076 (25 July 2284).
For the spin off series (Next Generation, Deep Space Nine, and Voyager), the writers marked out the season by using 5 digit stardates, starting at 41153.7 for the first episode of The Next Generation, increasing the thousands digit by one each season until arriving at 54973.4 for the final episode of Voyager. The first seasons of The Next Generation and Voyager also establish themselves to be set in 2364 and 2371, respectively. Since Mr. Main wrote the Stardate FAQ in 1996, the Voyager episode “Homestead” established itself to be on “First Contact day,” referring to the events in the movie Star Trek: First Contact, which occurred in the Star Trek timeline on 5 April 2063.
Assuming that “Homestead” takes place in April of 2378 means that the stardate rated is slightly slower than 1000 sdu/year. A rate of 33,000 s/sdu give us ~ 956 sdu/year. Working backward from “Homestead,” the two previous calendar year references are accurate. The last part is to link the films with the updated system. A happy medium is found at SD 17856.6 (19 May 2339) without changing the date of “Homestead.”
Note: This stardate calculator only corresponds to the Television series and the first ten feature films. The subsequent movies, led by JJ Abrahm's Star Trek, use a different system.