premature
Explanation
The Joke
A woman tells a man: "I should tell you, before we go to dinner, I'm a premature ejaculator." The man is confused and responds: "You don't understand. I do this prematurely. I have a negative amount of time." She clarifies: "Oh. All well, we can order turnips out for dinner then." In the next panel, we see that she has somehow already completed the act (represented by an embarrassed "Noooo!" from inside a house) -- and the scene reveals she already has two children and a starter home in the suburbs.
The joke is a play on the concept of "premature" taken to an absurd extreme. Rather than the typical meaning of premature ejaculation (which happens too quickly during a sexual encounter), this person's condition is so extreme that it happens before the date even starts -- in "negative time." The result is that she has already conceived children and built an entire suburban family life before dinner has even begun.
The Humor
The humor lies in the escalating absurdity of taking the word "premature" to its logical extreme. If something can happen too early, what if it happened so early that it preceded the event it was supposed to be part of? The comic pushes this to the point where the premature ejaculation has resulted in a fully established family -- two children and a starter home -- before the first date is even underway. The visual punchline of the suburban house with kids, combined with the anguished "Noooo!" scream, creates a perfect absurdist payoff. The gender reversal (a woman having this condition) adds an extra layer of surreal comedy.