The Earth’s rotation and speed changes with the weather and geological events. This means atomic time and solar time may not be identical. Sometimes leap seconds are needed for an adjustment. On 31 Dec 2016, a leap second was added, and the time 23:59:60 existed.
This practice has proven disruptive, particularly in the twenty-first century and especially in services that depend on precise time stamping for critical process control. The relevant international standards body has been debating whether or not to continue the practice.