How would you describe the semantic phenomenon that allows this joke?

Groucho Marx had a joke that's long been a favorite of mine:

I've had a wonderful time; this wasn't it.

I assume he's using the present perfect to say I've had a wonderful time.

But, when he tacks on this wasn't it does it change to past perfect?

Or was it past perfect all along because of the have had construction?

Solution 1:

(btw, there should be a full stop after wonderful time; otherwise it's a comma splice.)

Here's the way it works. There are four senses of the English Perfect Construction.
Normally the fixed phrase I've had a wonderful time would be interpreted in its Universal sense:

"used to indicate that a state of affairs prevailed throughout some interval stretching
  from the past into the present: I've known Max since 1960."

This means that from the start of whatever is going on (normally a party or other social event), the speaker has enjoyed themself, and thus this is a stereotypical leavetaking compliment to the host.

However, the next clause -- This wasn't it -- forces the Perfect into its Existential sense:

" used to indicate the existence of past events: I have read the book five times.

Therefore the implied compliment from the Universal sense is withdrawn, and the host is insulted.

Solution 2:

I've had a wonderful time; this wasn't it.

The joke works on the cancellation of a previously created implicature.

The first clause,

• I've had a wonderful time;

implies that the speaker (the "I") has just had a wonderful time -- that the most recent event was that wonderful time. (But this is not an entailment, and so could be explicitly cancelled, and if cancelled, it is usually cancelled almost immediately in the next clause or sentence by native English speakers.)

The next clause,

• this wasn't it.

cancels that previous implicature. Thus, the "joke".

I've noticed that many EFL speakers can't get the handle on this sort of creation and canceling of implicatures. (Aside: Especially the ones who think they know English grammar better than the native speakers, and they argue and argue, not understanding that what they are saying ain't quite what they think it is.)

EDITED to add more info:

One of the main differences between the present-perfect construction and the past-perfect is that the present-perfect explicitly includes the present time-sphere. Both types of perfect construction include the past time-sphere.

In the OP's example "I've had a wonderful time; this wasn't it", the verb construction of the first main clause has not changed -- it is still a present-perfect construction, which is used to talk about the past but with the understanding that the present is also important.

If the present wasn't to be included (present == this current social event), then the speaker would normally use the past-perfect ("I had had a wonderful time") which would explicitly omit that current/recent social event, but that would sound weird for this social situation, where it is assumed that a guest is thanking the host for a wonderful time at the host's event. And that assumption of "thanks" is needed for the joke to work.

If a guest wanted to thank the host, then the simple past would often work, "I had a wonderful time", as it could refer to the very recent past which could refer to the just concluded social event. But then the joke probably wouldn't work as well,

• I had a wonderful time; this wasn't it."

For the original joke with the present-perfect is explicitly including the present time-sphere, which strongly pulls in the implicature that the speaker is talking about the current social event as being that "a wonderful time".

The present-perfect has various uses, and one of them is to talk about a past situation that is very close to "now". Often, that situation is the present situation or a very recent situation. The joke relies on that common usage.

Solution 3:

"Brevity is the soul of wit," or so they say. If I suck out the brevity from Marx's witticism and add a bunch of words, it would look as follows:

"I've had a wonderful time in the past at gatherings similar to this, but this particular gathering did not qualify as a wonderful time."

The witticism is gone, obviously, but both "wonderful times" are in the past (viz., the near past and the not so near past).

Also, as Mitch, above, points out, the word had is used somewhat metaphorically to mean "[I have] experienced" or "[I have] been in the possession of" a good time. Consequently, the host of the party thinks initially to himself or herself,

"Oh, my guest is saying he has just had a good time at my gathering."

When he hears "this wasn't it," however, he then understands the insult, and thinks to himself,

"Oh, my guest has had a good time at a party other than mine, but he did not have a good time at my party."

They didn't call Groucho, Grouch-o, for nothing!