Count backward from any number to zero to indicate time, as in The final seconds before take off were counted down: 10, 9, 8, etc. The down in this term refers to the decreasing size of the numbers. This usage originated in connection with the firing of missiles and spacecraft and has since been broadened to such events as the roll call of states at a political convention. [c. 1950]

Make final preparations for an event, as in Hong Kong was counting down to the day when it became part of mainland China. This usage is derived from def. 1. [Late 1950s]