If the country’s experts were to use every centrifuge, they could process this material into enough weapons-grade uranium for one nuclear bomb in about one month and 20 days, according to the latest analysis. Of course, they would almost certainly be caught by the IAEA, which monitors Iran’s nuclear plants – and building a warhead for a missile would take at least another year. There is, furthermore, no evidence that Iran’s leaders have decided to embark on such a perilous course. But every time they install a new cascade of centrifuges, or enrich another kilogram of uranium, the time they would need to “break out” to build a nuclear weapon shortens. Hence Mr Kerry’s anxiety to “stop the clock”.