It's bad writing, Johnny; and we know the distances of planets can determined different cycles of time and months or ever years. The distances of space travel would alter any sort of relevant Earth order of time
Well, I agree with you in principle that different planets have different planetary years, of course, but since two different characters in the movie plainly tell us that it's been 15 years since Khan was marooned on Ceti Alpha V, we're forced to assume that either Khan was keeping track of the Earth years for some reason (which would fit with his 20th century two-dimensional thinking in the final battle), or that the years of Ceti Alpha V's altered orbit were close enough to Earth years as to make no substantial difference.
It's either that, or we assume that
both Kirk and Khan didn't know what they were talking about, and that's not something I'm prepared to do.
And
Trek has never really alluded to time passing differently on Earth than it does aboard the
Enterprise, probably because they knew it could confuse the hell out of casual viewers
. I suspect that warp speed was partially a way to sidestep those issues.
I only take what was seen and heard on TOS as facts
So I assume that you're putting TOS on a higher pedestal than the movies when it comes to continuity, then?
I tend to look at this question from a character point of view. Khan's basic characteristic in TWOK is that he's obsessed with Kirk and blames him for all the hardships he's had to endure on CAV. Khan would absolutely know that it's been 15 years he's spent on CAV. Hell, he probably had it timed down to the second, because he was obsessive, crazy, and he didn't have much else to do outside of survive and brood. And since Khan was a genius in his own right, I wouldn't put it past him to calculate the time differential. That seems like something that would be within his abilities.
But Kirk confirms that it's been 15 years since he last saw Khan, and he ought to know. I would assume that Kirk likely called up his old logs of the
Enterprise's encounter with the
Botany Bay while they were en route to Regula I to refresh his memory (offscreen, of course). They never state so in the movie, but Kirk's a smart guy and it seems a prudent thing to do.