Countdown

A countdown was a way of telling time in which the goal was to count backwards to zero. At the point that the clock reached zero, any number of things could happen, such as vehicular launch (TV: Utopia, The Ambassadors of Death, The Impossible Astronaut) or the detonation of a bomb (TV: Empress of Mars) or self-destruct mechanism. (TV: A Town Called Mercy, Journey to the Centre of the TARDIS)

The Twelfth Doctor said he never could resist a countdown. He joined in on a countdown chant at NASA, for the oncoming transmission from Mars probe Valkyrie. (TV: Empress of Mars)