The first question of the vice-presidential debate on Tuesday night was whether the candidates would support or oppose a pre-emptive strike on Iran by Israel. It framed the issue for Gov. Tim Walz and Senator JD Vance as urgent to consider because Tehran has “drastically reduced the time it would take to develop a nuclear weapon,” cutting its acquisition time to “one or two weeks.”

The premise behind the question from Margaret Brennan of CBS News, one of the debate’s moderators, highlights a popular confusion over what it takes to build a usable nuclear bomb.

Nuclear experts said on Wednesday that it would take Iran not weeks to make a nuclear weapon, but months and possibly as long as a year. Ms. Brennan’s question, they added, began the debate on a false note.

“I don’t think there’s a danger that Iran this year is going to start exploding nuclear weapons,” said Houston G. Wood, an emeritus professor of mechanical and aerospace engineering at the University of Virginia. A specialist in atomic centrifuges and other nuclear matters, Dr. Wood estimated that it would take Iran up to a year to devise a weapon once it had enough nuclear fuel.

“It would likely take many months,” said Siegfried S. Hecker, a former director of the Los Alamos weapons laboratory in New Mexico, “not weeks.”

The experts said that the CBS question conflated the time it would most likely take Iran to manufacture a bomb’s worth of highly enriched uranium with the overall process of turning it into a weapon. Once enough uranium metal is produced, it must be carefully machined into the core of an atom bomb, which is then set amid the other parts of a nuclear warhead that would sit atop a missile.