How was the Star Trek timeline officially established?

The Original Series is set from 2265 to 2269, but as far as I can remember, the year was never given until much later, in The Next Generation, which arbitrarily started 100 years later.

Somehow, someone, most likely Mike Okuda or Gene Roddenberry himself, decided to make 2265 the start year, but how was that decided? What specific clue or line tied it down to that year? Perhaps in the last season of TOS someone said onscreen that it was 300 years since Apollo 11?

Answer

It seems my comment above was incorrect on two accounts. First, the direct answer to the question. On the Memory Alpha page for Stardates is an excerpt from an interview with Gene Roddenberry, including these lines:

In the beginning, I invented the term "star date" simply to keep from tying ourselves down to 2265 A.D., or should it be 2312 A.D.? I wanted us well into the future but without arguing approximately which century this or that would have been invented or superseded.

The book it appeared in was thought up in May 1967 and published in September 1968 - so Roddenberry had the year 2265 in his mind as early as Season 2 of The Original Series.

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Here's the other part, that surprised me, from higher up on the same page: Apparently the fact checker had his own opinions on the very first episode. He thought "Stardate 1312.4" should instead be renamed to "Julian B 1312.4", a time system based on the real-life Julian days. That particular date would translate to August 5, 3271.

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The script writers may well have used that Julian day system to come up with the inconsistent "200 or 1200 years in the future" that TOS kept falling to. Which sort of worked to Roddenberry's advantage, because as he said in the interview, he didn't want to quibble over the exact year.

_{(And I have to say, they kept the year confusing pretty well!)}