In TOS there are pilote-episodes. Special among these is "Where No Man Has Gone Before" with other uniforms, no DrMcCoy but a chief medical officer Dr. Mark Piper and the physicist Sulu. In "Advance Information on 1966-67 Programming Star Trek by NBC Television Network" we can read "Chief Medical Officer Dr. Piper is the oldest and most experienced space traveler aboard the Enterprise" and about "Science Officer Spock", who "has a precise, logical turn of mind inherited from his father (a native of the planet Vulcanis, who married an Earth women)". There is also the "Engineer Officer Scott", "Communications Officer Alden", "Physicist Sulu", who "is the trim, sofrt-spoken chief of the Enterprise's Astro Science Department" and "Yeoman Smith, who has drawn the important assignment of secretary to the Captain".

In the following 1st season of TOS Sulu primary duties included manning the helmsman station, and the firing of the ships weapons as shift tactical officer. And Lt. Uhura serves as Communications Officer. The new personal help for our Captain is Yeoman Janice Rand. And also new on board is Dr. McCoy. (see TOS: The Corbomite Maneuver).

There are three seasons of TOS and two seasons of TAS (with a very short 2nd season). All together they are the 5 year mission, which Kirk completed sucessfully and returned to become Admiral (see Star Trek I: The Motion Picture).

It seems to be logical, that the "one season is one year" system of TNG was already true for TOS:

• The Enterprise starts her mission. In a period of a half year there is somewhere the pilot "Where no man has gone before".
• First season TOS with some changes on the crew and with new uniforms represents one year.
• Second season TOS also a year.
• Third season TOS = one year.
• First season TAS = one year.
• Second season TAS, which is very short, represents the rest of the five-year-mission. Therefore it is a half year.

In the 6th movie McCoy answered in the Klingon court-room, that he served as ship's surgeon and chief medical officer for 27 years.
At first it is to say, that he was the ship's surgeon of both ships: the USS Enterprise (NCC-1701) and the USS Enterprise (NCC-1701-A).
On the other hand he retired after the historic five-year mission to enter private medical practice.
two and a half years later, McCoy's commission was reactivated by a "little-known, seldom-used reserve activation clause" at the request of now Admiral James T. Kirk (see STM: Star Trek I.)

He was the chief medical officer on the Enterprise in the 1st season TOS (but not in the TOS pilote episode). If the 2nd season means 2265 or 2266, we have to think, that Bones joined the Enterprise 2264 or 2265.

Adding 27 years and 2,5 years we get the date of the 6th movie.

• Therefore the "Khitomer peace conference" was between 2293 or 2295.

But there are some more important hints in StarTrek VI: The Undiscovered Country:

• We can hear from the captain's log, that Captain Hikaru Sulu took command of the Federation starship Excelsior for a three-year mission with a focus on charting gaseous anomalies in Beta Quadrant.
• But it was not said, that this was the first mission! He is "more than 10 years Captain of the Excelsior" (p. 31) Before he served on the USS Enterprise-A after a year of service. (p. 19).
• Praxis exploded two month before the meeting in StarFleet-Command with Kirk took place.
• Kirk's son David died 11 years ago. (p.42)
  Kirk's son died more than ten years ago. (p. 65)
  Kirk talked to Carol 1 year after David's dead. Carol tried to talk to him before, but Kirk was on a mission (p. 10)
• Kirk's talk with Carol happend 10 years ago (p. 10)

Therefore StarTrek III with the death of David happened 11 years before, therefore sometimes 2281 and 2284. And StarTrek II was the direct prequel to StarTrek III. There we can see a bottle of ale from 2283. Therefore we must be in the year 2283 or 2284 (Someone believe, that McCoys "It needs to age" tells us, that we 2283 is far in the past - but it could also be mean: "Don't forget, Jim: It's more fine, if it could age")

About StarTrek II. and the direct prequel StarTrek III.:

• Kirk's statement that he hadn't seen Khan for "15 years":
  Khan Noonian Singh is found in TOS 2. season "Space Seed" - also during that season the Babel-conference took place, which means 2265 or 2266.
  "15 years" could be read "between 13 and 17 years". Therefore Star-Trek's second movie would play sometimes between 2278 and 2283. With the statement above we can sure, that it is 2283. And TOS 2nd season is 2266.

• In ST III, which follows directly the storyline of ST II, Admiral Morrow states that the "Enterprise" is 20 years old. It is often said, that the dates from StarTrek II. are "suspect and should be discarded". Why? I don't know. I could not find a reason for that. See for yourself:
  Maybe Admiral Morrow was referring to the age of the upgraded "Enterprise", which happened between Pike and Kirk, everything would be allright. Maybe it was the Mater/Anti-Mater-Engine, of which Kirk was proud of in the novelation of ST I ("The Annihilation subject mass only of a pin's head size propelled the Enterprise with just as much energy like it by thousand old hydrogen fusion engines produced is.")
  

If TOS 2nd season means the year 2266, Kirks mission started 2264. And Kirk's (!) Enterprise would be 20 years old!

Result:

• The Enterprise starts her mission. In the second half of 2264 are the events of "Where no man has gone before".
• First season TOS = 2265
• Second season TOS = 2266
• Third season TOS = 2267
• First season TAS = 2268
• Second season TAS and end of the Enterprise's mission under James T. Kirk = 2269.Following her triumphant return, Kirk was promoted to Admiral and posted to Chief of Starfleet Operations. An eighteen-month refit follows, before Kirk takes the Enterprise to meet Vīger.
• Star Trek I. = 2271 or 2272
• Star Trek II. = 2283 or 2284

• That is why Kirk must have been born (2266-34=) 2231 or 2232.
• "Where No Man Has Gone Before", TOS-pilot-episode, is laid two years before TOS second season!
• First season TOS is between this pilot and the second season.

In "Where No Man Has Gone Before", TOS's true first pilot-episode, we read on the tombstone for 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