He was also 21st-century astronaut John Kelly, a Klingon, and a Jem'Hadar.
Also, it would be more like 15 1/2 years, since it's no more than a few weeks after The Wrath of Khan and "Miri" was about half a season before "Space Seed." Although by the official chronology it's 18 years for some reason.
18 years was a sort of a compromise.
Star Trek II, III, IV, & V should all happen within a few months of fictional time.
In
Star Trek II: The Wrath of Khan, Khan and Kirk both say it is 15 years since they saw each other. And presumably the last time they saw each other was when Kirk left Khan on Ceti Alpha V, shortly after the end of "Space Seed".
The treaty that established Nimbus and the first settlements on it by people from the Federation, Klingon, and Romulan realms had to be later than "Balance of Terror", when nobody knew what Romulans looked like. After Romulans and avarious Romulan subjects began settling Nimbus III and mingling with Federation citizens, the fact that Romulans looked like Vulcans would soon become public.
In
Star Trek V: the Final Frontier Caitlin Dar said that Nimbus III was established 20 years earlier.
"Balance of Terror" and "Space Seed" were both during the first season, which lasted about one to two years in most versions of
Star Trek chronology.
Star Trek II: The Wrath of Khan and
Star Trek V: the Final Frontier should happen only a few months of ficitonal time apart. But
Star Trek II: The Wrath of Khan is said to be 15 years after "Space Seed" and
Star Trek V: the Final Frontier is said to be at least 20 years after "Balance of Terror".
So the official chronology splits the difference and also tries to spread out those four movies over three calendar years.
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