[Nate the Great]

We Travel By Night tackles the TOS->STTMP period, which I call The Lost Years after the novel subseries set during that period.

In STTMP Kirk states that he's been in the admiralty in Starfleet Operations for 2.5 years, yet Decker states that the 1701 was being refit for only 1.5 years. So what was going on in that missing year?

To me the simplest solution is that Spock was using the Enterprise as a training vessel just like he would do prior to TWOK.

Then again, I'm dubious that you could strip a starship down to its skeleton and rebuild it in the 23rd century in just 18 months.

And of course you have to ask why Starfleet would bother replacing 90% of a ship in the first place. Just make the STTMP ship the Enterprise-A and be done with it! (Of course, that would push the TNG ship out to F at least, but that's a discussion for another day).

For that matter, why was Decker in charge of the refit? The possible explanations don't really stand up to scrutiny:
1. It was tradition for the next captain to be in charge of the refits between missions, and Kirk did this before TOS. I don't really like this, as starship design, construction administration, and starship command are different skillsets.
2. Decker just happened to be trained in both and he asked to be allowed to be in charge of the revit. Possible nepotism what with the loss of his father back in "The Doomsday Weapon".
3. This was some sort of scheme by Decker to "earn" the captain's seat as the next step in his career.