Not sure about 15 years after TMP but both Kirk and Khan repeat the line about it being 15 years since they last met. Most assume this means the events of
Space Seed but I've read at least one chronology which postulates a second brief trip to CA5 where Kirk dropped off additional building supplies for the colony, accompanied by Mr Chekov (to close off that plot hole).
So, TWOK is definitely 15 years after
something
You would have to shoe-horn that second visit into the six-months before CA6 exploded.
Khan: Ceti Alpha Six exploded six months after we were left here.....Admiral Kirk never bothered to check on our progress.
(granted, how long is a CA5 month but I would expect use of a standard UFP calendar until you work out the CA5 one.)
In that interpretation, the 6 months would be after Kirk left (i.e. abandoned) them there the second time. Since Kirk came back after delivering them there after Space Seed, Khan would not consider that in the same light.
That's a stretch but I have no real issue with it.
It's not a theory I'm totally wedded to, but it provides interesting food for thought!
There is another bit of evidence that supports that theory.
In the first season episode "Balance of Terror", stardates from 1709.2 to 1709.6:
SPOCK: Referring to the map on your screens, you will note beyond the moving position of our vessel, a line of Earth outpost stations. Constructed on asteroids, they monitor the Neutral Zone established by treaty after the Earth-Romulan conflict a century ago.
[Sickbay]
SPOCK [OC]: As you may recall from your histories, this conflict was fought,
[Engineering]
SPOCK [OC]: By our standards today, with primitive atomic weapons and in primitive space vessels
[Bridge]
SPOCK: Which allowed no quarter, no captives. Nor was there even ship-to-ship visual communication. Therefore, no human, Romulan, or ally has ever seen the other. Earth believes the Romulans to be warlike, cruel, treacherous, and only the Romulans know what they think of Earth. The treaty, set by sub-space radio, established this Neutral Zone, entry into which by either side, would constitute an act of war. The treaty has been unbroken since that time. Captain.
And when Spock gets a visual image of the Romulan control room:
SPOCK: I have a fix on it, Captain. I believe I can lock on it, get a picture of their Bridge.
KIRK: Put it on the screen.
(Up shimmers an image of a group of four humanoids around a console. One leaves his post and salutes the figure with his back to us. That figure then turns, and we see someone who looks just like - a Vulcan. Both Spock's eyebrows hit the ceiling. There's a long silence and a lot of stares.) KIRK: Decoding?
No one knew what Romulans looked like, and so everyone is surprised to see that Romulans look like Vulcans.
"Space seed" is also in the first season, and has stardates from 3141.9 to 3143.3. At the end Kirk offers Khan and McGivers the chance to settle on Ceti Alpha V - apparently Kirk thinks the other 72 supermen don't deserve a chance to decide for themselves.
In
Star Trek II:The Wrath of Khan, about stardate 8130.4:
KHAN: You are in a position to demand nothing, sir. I, on the other hand, am in a position to grant ...nothing. What you see is all that remains of the ship's company and crew of the Botany Bay, marooned here fifteen years ago by Captain James T. Kirk.
And:
KIRK: There's a man out there I haven't seen in fifteen years who's trying to kill me. You show me a son that'd be happy to help him. My son. ...My life that could have been, ...and wasn't. And what am I feeling? ...Old. ...
So Kirk and Khan both think that they haven't seen each other for fifteen years. So
Star Trek II:The Wrath of Khan should be 15 years after the last meeting of Kirk and Khan, which should have been either soon after "Space Seed" ended or else during any hypothetical second visit of Kirk to Ceti Alpha V that might have happened.
The next three movies all seemed to happen within a period of less than one year. And in the last of of those movies,
Star Trek V: The Final Frontier, a new Romulan representative arrives on Nimbus III, the Planet of Galactic Peace:
CAITHLIN: Gentlemen, I'm Caithlin Dar.
TALBOT: Ah, yes. Our new Romulan representative. Welcome to Paradise City, my dear, capital of the so-called 'Planet of Galactic Peace.' I'm St. John Talbot, the Federation representative here on Nimbus Three and my charming companion, here, is the Klingon consul Korrd.
KORRD: Ugghhhh!
CAITHLIN: I expect that's Klingon for hello.
TALBOT (OC): Won't you come in, my dear?
(Sybok and his followers approach Paradise City)
CAITHLIN: Twenty years ago, our three governments agreed to develop this planet together. A new age was born.
TALBOT: Our new age died a quick death. And the settlers we conned into coming here, they were the dregs of the galaxy. They immediately took to fighting amongst themselves. We forbad them weapons, but they soon began to fashion their own.
CAITHLIN: Right! Then it appears I've arrived just in time.
It sees logical to assume that the treaty establishing Nimbus III must have been signed after "Balance of Terror". And it seems certain that settlers would have begun to arrive on Nimbus III after "Balance of Terror", because any subjects of the Romulan empire who settled there would probably soon describe what Romulans looked like to people who asked them.
Even thought the fictional time span between
Star Trek II:The Wrath of Khan and
Star Trek V: The Final Frontier was less than one year, they were released five years apart, in 1982 and 1989 respectively.
Star Trek II:The Wrath of Khan was filmed in 1981, about 15 years after first season episodes like "Balance of Terror" and "Space Seed" were filmed, and
Star Trek V: The Final Frontier was filmed in 1988, about 22 years after first season episodes were filmed.
So the makers of
Star Trek V: The Final Frontier might have assumed that since it was filmed 22 years after first season episodes were filmed, it should have a fictional date 22 years after the fictional date of the first season episodes. And I can't help thinking that it was really amateurish of them not to count the fictional time instead, and realize that
Star Trek V: The Final Frontier should be no more than 16 years after "Space Seed" in fictional time.
One possible solution would be that Caitlin Dar was using Romulan years in
Star Trek V: The Final Frontier and they were shorter than the years used by Kirk and Khan in
Star Trek II:The Wrath of Khan so that 20 of Dar's years equaled about 15 of Kirk and Khan's years.
And another possibility is that Kirk, Khan, and Dar used years of the same length. In that case Nimbus III would have been established sometime after "Balance of Terror", and possibly before "Space Seed". Kirk would have visited Ceti Alpha V a second time at least 5 years after Nimbus III was established.
Star Trek II:The Wrath of Khan would have happened 15 years after Kirk's last visit to Ceti Alpha V and 20 years after Nimbus III was established and even longer after "Balance of Terror". And
Star Trek V: The Final Frontier would happen less than a year after
Star Trek II:The Wrath of Khan.
So if Kirk and Chekov made a second visit to Ceti Alpha V about 5 years after "Space seed", that would explain why the four
Star Trek movies in the 1980s could happen less than one fictional year apart and both 15 years after Kirk & Khan last saw each other and 20 years after Nimbus III was established.