How long would you say it would take to build the Enterprise?
The only benchmark we have for this is the line from TMP. Considering that it took "18 months to redesign and refit the Enterprise" (according to Scotty... though I'm certain that major elements were "redesigned" LONG before that point!)... I'd be inclined to say that it would be between 12 months to 18 months to build an Enterprise-class starship (aka a "Constitution-refit class starship").
If there were only tried-and-true design features present, it would be closer to 12 months. If there were new technologies and new practices which needed to be developed, 18 months is more like it.
Obviously, to build a ship like that today, using modern techniques, it would be more like six or seven years, all told, I'm sure. But we have to allow for more advanced technologies in the future, don't we?
By comparison... how long has it taken to "redesign and rebuild" buildings on the World Trade Tower site, so far? How long will that take, overall, do you think? And how much simpler is a building like that, compared to a starship?
I must reply to the comment from above where someone apparently willfully misinterpreted my comment about Morrow "playing Santa Clause" as somehow "playing a joke." I said no such thing.
In fact, the "reveal" at the end of ST-IV was EXACTLY the same sort of thing I was suggesting... a "happy surprise."
If, after the Mutara Nebula battle with Khan, Starfleet was send damage reports on the ship's condition, and if they has a number of new, improved technologies which they wanted to implement, and if the hull of the Enterprise was in such terrible shape that it would be almost as costly (time and resources) to repair it as to build a new ship... what do you think they'd do?
And consider... the Enterprise was ALREADY assigned as a training vessel... no longer on duty as a "ship of the line." Meaning... it had probably seen enough damage through it's career that it was considered largely compromised already.
Approval of a new-build Enterprise-class hull... within a week of the realization that the Enterprise herself wasn't worth repairing... would be pretty easy to get, I'd think. Especially considering that another ship of the line had also been lost in that encounter, and the Starfleet was measurably weakened as a result.
So... here's a timeline...
(1) Enterprise damaged at Mutara Nebula. Spock dies. Report made to Starfleet Command.
(2) Enterprise stays on-station at "Genesis" for a while, until a security picket can be set up to secure the site. A "tender" arrives to help Enterprise restore basic operational status. David and Carol Marcus are escorted to Starfleet Command and the Federation Counsel by Saavik.
During this period, SOMEBODY rescues the crew of the Reliant from Ceti Alpha V. Maybe the Enterprise is the only ship in the "quadrant" available to do so?
(3) The Enterprise "limps" back to Earth. Scotty rigs a slave-circuit to the warp drive system... no real justification for that is given, of course, but it makes no sense unless the ship is not likely to see a full crew reassigned anytime soon, does it? It'd estimate that this flight takes at least three weeks...
Upon arrival at Earth, Kirk is informed that the Enterprise will not be repaired, because "we feel her day is over." It is VERY unlikely that this means that she was intended to be replaced by an Excelsior-class ship, since the Excelsior-class is still "experimental" at this point. I suspect that a replacement ship (name TBD?) was already underway, but far from completed, at this point. (Perhaps it was going to be the Ti-Ho or whatever...)
At least a couple of months would pass. I don't accept that the "on-screen time" must equal the time of the real events. I'm sure that, from the time Enterprise limped back in, to the time Kirk and Co. "stole" her, that at least two months passed. Your mileage may vary...
So... we're up to about four months from the Mutara Battle to the theft of the Enterprise.
The Enterprise is still "limping" and thus would take a significant amount of time to get back to Genesis... let's say that it's the same as it took for the ship to get back to Earth... roughly three weeks. (Clearly, the crew just kept wearing the same clothes every day... hopefully 23rd-century laundry works fast!) Almost five months...
Now... the crew captures a Klingon BOP. How long does it take for them to figure out how to handle it? How long does it take for them to get to Vulcan? Shall we say at least a week to get the ship under their control, learning what goes where and why? And let's say another three weeks to get to Vulcan.
That's about six months from the Mutara Battle. Now... three months land-locked on Vulcan, means nine months from the Mutara Battle to the beginning of ST-IV. Let's say that it's two weeks to get to Earth from Vulcan (ignore the ST'09 "hyperspace" version of Warp Drive, and stick with the better-accepted Star Trek version).
The whole of ST-IV is "instantaneous" from the 23rd-century perspective. But imagine what comes next!
- debriefing on the Klingon ship.
- arrest (even if only "obligatory") for the theft of Enterprise
- a trial... we only see the final verdict part, but think of how long the Casey Anthony thing went on...
- some period of "post-release" settling-in.
I have no problem imagining that the whole period, from the crash-landing in SF bay to the end of the trial might be four to six months. Let's say it's four months, to be fair.
This takes us up to 13 1/2 months from the Mutara battle.
Now, they might have been given "relief duty" for a period of time as well. Spock, in particular, would be given EXTENSIVE medical and psychological testing prior to being allowed to resume a position of responsibility on a starship, don't you think?
So, is 18 months really unreasonable, from the Mutara battle to the "unveiling" of the Enterprise A?
"Film time" need not bear much, if any, relationship to "real, in-universe - albeit fictional - time."