A time lock, is often, quite literally a clock, counting down to zero before the bomb explodes. Examples abound: In Kubrick’s Dr Strangelove, the pilots flying the aircraft carrying an atomic bomb that will start the next world war have to be persuaded not to drop it before they reach the target site; in Next, Nicholas Cage has to foil the villain’s plans before a nuclear device wipes out LA.
In my own novel, Scarab II: Reawakening, the Hero, Jack Wheeler, has to get to the quantum computer before the appointed time to stop it from running the programme that may destroy the world.
In 36 Hours the time lock is even more tightly woven into the story’s structure. With the invasion of Europe but days away, the Nazis have little time to extract the date and landing location of the Allied forces from James Garner. Although the story might still work by concentrating on how Garner is seduced into talking, the time lock imbues the story with an overall tension that could not be achieved otherwise.
In Armageddon, a shaft has to be drilled and a bomb placed deep into the comet that is headed for earth…
In The Bridge on the River Kwai, not only must the bridge be built under the most trying circumstances, but it has to be finished by a specific date. The highlight, which shows not only the bridge being completed at the ninth hour just as the train arrives, but also in time for the explosion to occur that sends both bridge and train crashing into the river, has rarely being surpassed in effectiveness.
A time lock in a story defines a specific time period for the main story goal to be achieved in order to avoid calamity or failure. The device increases tension and helps to maintain the forward thrust of the story.