The thing that threw me was when Harry Morrow said
"Jim! The Enterprise is 20 Years old! We feel her day is over."
Now I was never under any illusion that the Motion Picture was 20 years before Wrath of Khan as Space Seed only occurred (according to dialogue onscreen) as 15 year before!
Pure conjecture on my part ...
Twenty years before Admiral Morrow's comments in TSFS, the Enterprise underwent major modifications even more extensive than those in TMP. Less on the exterior, more on the interior. Morrow was dating the Enterprise not from the date of her original launch, nor from the "refit," but from that event.
This would have been a few years prior to Kirk assuming command.