The clocks striking thirteen is also a reference to the militarization of society which developed during the Second World War.
Military time use a twenty-four hour clock.
Anthony Burgess, 1985. Little Brown, 1978. There are so many different editions of Nineteen Eighty-Four that all citations from it will be by section and part number. Regnery Gateway,1987, c. 1943. 46 and 82.