In "Where No Man Has Gone Before", TOS's true first pilot-episode, we read on the tombstone for James T. Kirk, that he was born 1277.1 and should die at 1313.7; that could be understood, that Kirk should die in this episode being 34.6 years old.
But we have seen, that Kirk was 34 years old in the 2nd season TOS: The Deadly Years, which was 2266.Therefore this idea is nonsense - stardates could not to be calculated like arithmetic dates.

This time system adjusts for shifts in relative time which occur due to the vessel's speed and space warp capability. It has little relationship to Earth's time as we know it. One hour aboard the U.S.S.Enterprise at different times may equal as little as three Earth hours. The stardates specified in the log entry must be computed against the speed of the vessel, the space warp, and its position within our galaxy, in order to give a meaningful reading.

Gene Roddenberry

The beginning of TOS (the pilot or all three pilots) happened therfore 2264. After the change of the uniforms we are in TOS real 1st season and in the year 2265. Following that TOS 3rd season is in the year 2267.

Without the exeption of the pilot the same system was used by Gene Roddenberrys Team for The Next Generation.

When TNG came along somebody - presumably Gene himself - decided to have a much more sensible, logical system of Stardates. To show that the new series was set later on than the original, all Stardates were five digit numbers rather than four. The first digit was a four, since the show was set in the 24th century. The next digit was a 1 since it was the first season of the show. And every season was a new year on Earth. And towards the end of the first season, Data gave us the year as being 2364.