TOS is fairly consistent within itself and just about
holds together. It's only with later incarnations that problems start to arise, beginning with TMP:
DECKER: NASA. National Aeronautics and Space Administration. Jim, this was launched more than three hundred years ago.
That would put TMP into the 2280s at least
, assuming the Voyager series continued (which it didn't of course)
The most consistent TOS dating puts it in the early years of the 23rd Century.
Unless Decker was just lousy at history?
You're being too forgiving. TOS is not
consistant with itself. At all. References dating the show vary from 200 to 900 years in our future, for starters. The design of the ship changes from shot to shot (when the pilot footage is reused). Yet as a fan you let it all slide.