^Yeah, but that Nimbus III dating is based on the assumption that the speakers meant exactly 20 Earth years, rather than rounding up or using alien years.
Yeah, they certainly could've been rounding up, but a "20 years" figure definitively puts Nimbus III's founding into the TOS years. It doesn't make a great deal of difference if it was 2267 or 2269, as long it was sometime after the Romulans reemerged in "Balance of Terror." Heck, the film
First Contact adjusted the date of Cochrane's first warp flight from the
Chronology's date of 2065 to 2063, and it didn't make a great deal of difference either way. The Okudas were smart enough to realize that it more or less amounted to the same thing if they took every round figure literally or if they didn't.
And I'm not a big fan of the "Oh, but they were using
alien years" chronology theories. That just seems to be a convenient excuse to ignore numbers whenever an alien is speaking. If their dialogue is translated into English for us, I see no reason why the units of time shouldn't be as well.
It's contradictory that they assumed ST V's "20 years" reference had to be exact but ignored ST II's "15 years" reference. It would've made more sense to do it the other way, or struck a compromise in between them.
Yeah, changing the time gap in TWOK from 15 years to 18 really makes no sense, especially since it had been 15 years in real life (1967 to 1982) and both Khan and Kirk confirm that it had been 15 years since Khan was exiled to Ceti Alpha V. I can see Kirk not remembering how long it had been, but
Khan? Obsessive, super-smart, maniacal Khan, who had next to nothing to do for 15 years but obsess over James T. Kirk and how he was responsible for all of Khan's ills? No way.