But 15 years is a round figure, while 18 is not. The former could mean the latter, even if not vice versa.
...Anyway, TWoK had to be in 2284 or 2285 if not later, because of the date on the Romulan ale bottle. Clearly, that bottle was several years old, or otherwise McCoy's and Kirk's lines regarding it would make zero sense. Any attempt to place TWoK earlier than 2284 would go against writer intent.
Which as such isn't necessarily a bad thing, but in this case it wouldn't really help with anything. The movie seemed to adhere to the "airdate plus three centuries" thing that apparently had been the working assumption from TMP onwards, and seems to have been a popular fan interpretation of TOS as well during the intervening years. TWoK should thus be rather immobile, and the 15-year thing would be achieved by moving TOS to a later date - but that would ruin the underlying systematic, easy-for-the-writers approach, whereas the "Kirk rounded it down so that it wouldn't sound so bad" thing would preserve it.
Dagger of the mind in seaon 1 seems to be set after christmas then they have Miri and the conscience of the King squeezed in before the end of the year
That's one of the things that works better in stardate order. Only "Conscience" would be between "Dagger" and the new year in that order, and that presents few if any problems.
However, if one goes by the stardates, one is then also tempted to treat them like TNG era stardates: that is, a date XY000 would correspond to the end of summer and beginning of season, while a date XY999 would correspond to the end of the season. In that case, "Dagger" comes in early spring which is perfect, and there's no rush regarding the following episodes. That theory spreads out the TOS episodes across the whole five years, without really affecting any of the conventional anchorpoints...
Naturally, the Okudas didn't put much weight on stardates, which indeed were arbitrary back then. But by sheer coincidence, treating the TOS stardates as non-arbitrary solves half a dozen TOS chronological contradictions and oddities while creating essentially none in return. (There's one real case of stardate overlap there - but we know Kirk jumped back and forth in time during the early years of his mission, so...

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Timo Saloniemi