When the Clocks Changed to Atomic Time
On 1st January 1972, the definition of the second changed from being based on the rotation of the Earth to being based on the Caesium Atomic Frequency Standard.
The switch from astronomic time to atomic time (which had to take place exactly at midnight) was a nerve-racking occasion for all involved. A double pulse code had to be introduced to allow astronomic time (to the nearest 100 millisecond) to be extracted from newly radiated atomic time signal. The task was completed successfully and the New Year was toasted in with a bottle of fine whisky.
The switch to atomic time meant that, from then on, leap seconds were introduced to keep atomic time approximately in sync with astronomic time (Earth time).