zero hour

plural zero hours

1. Synonym of H-hour.

   I waited with occasional glances at my watch until the zero hour should arrive.

   1929, Arthur Conan Doyle, When the World Screamed

   2 more examples

   Zero hour was forty-five minutes after midnight.Two companies were to attack on a 600-yard front […].

   1937, Siegfried Sassoon, The Complete Memoirs of George Sherston, 1972 paperback edition, London: Faber, page 339

   She packed my bags last night, pre-flight / Zero hour 9 a.m. / And I'm gonna be high / As a kite by then

   1972 April 17, Elton John, Bernie Taupin, “Rocket Man”, in Honky Château, performed by Elton John

   Synonyms: H-hour