One would certainly assume that Khan would be speaking of his years, not anybody else's. Thus, if Ceti Alpha V had identifiable seasons, fifteen cycles of those would be what Khan considers 15 years - the timekeeping systems of the rest of the galaxy would not concern this egomaniac.
The same would apply to the 1990s being 200 years in his past. Between 1996 and 2266, he probably only slept through two centuries, not closer to three, what with traveling in a relativistic-speed spacecraft and all that.
Kirk in turn would be likely to round the true figure down, not up - no matter what the official story, he'd feel guilty about neglecting his former buddy Khan for so long, and even 18 years of neglect would then be rounded to 15 rather than 20.
Timo Saloniemi