If a nuclear missile is launched at the United States the President has just 6 minutes to come to terms with that and decide to launch a counter attack or not.

US nuclear deterrence in 2024 doesn’t rely on launch-on-warning, but on the expectation that no hostile power has the ability to locate and destroy the US ballistic missile fleet prior to them performing their counterlaunches.

https://en.wikipedia.org/wiki/Second_strike

In nuclear strategy, a retaliatory strike or second-strike capability is a country’s assured ability to respond to a nuclear attack with powerful nuclear retaliation against the attacker. To have such an ability (and to convince an opponent of its viability) is considered vital in nuclear deterrence, as otherwise the other side might attempt to try to win a nuclear war in one massive first strike against its opponent’s own nuclear forces.

Submarine-launched ballistic missiles are the traditional, but very expensive, method of providing a second strike capability, though they need to be supported by a reliable method of identifying who the attacker is.

https://en.wikipedia.org/wiki/Launch_on_warning

Launch on warning (LOW), or fire on warning, is a strategy of nuclear weapon retaliation where a retaliatory strike is launched upon warning of enemy nuclear attack and while its missiles are still in the air, before detonation occurs.

In 1997, a US official stated that the US had the technical capability for launch on warning but did not intend to use a launch on warning posture and that the position had not changed in the 1997 presidential decision directive on nuclear weapon doctrine.

This is also true of the French and British nuclear deterrents, FWIW – the British don’t even maintain a nuclear arsenal other than on subs, so they haven’t even bothered with maintaining the option to do so, and the French only use tactical ALCMs in addition to the strategic sub-launched weapons.