The international standard of time is Coordinated Universal Time (UTC), which is used in countries around the world almost exclusively as the basis of civil time. UTC is derived from atomic clocks located in the National Physics Laboratories of various countries and is based on the international definition of the second. However, the Earth takes slightly longer than 24-hours, or 86400, seconds to complete one full rotation. Therefore, very slowly, UTC slightly drifts away from time references based on the rotation of the Earth (solar time standards). Additionally, the rotation of the Earth can speed-up or slow-down slightly, it is well-known that some big Earthquakes can change the speed of rotation of the Earth, slightly shortening days.
In order to compensate for the divergence of UTC from solar time, leap seconds are occasionally introduced. A positive leap second is inserted at 23:59:59 of a specific day to increase the length of the day to 86401 seconds. A negative leap second is achieved by removing a second before 00:00:00.
Example of positive leap second insertion:
23:59:60 – positive leap second insertion
Example of negative leap second removal:
23:59:58 – negative leap second removal
Due to variations in the Earth’s rotation, positive leap seconds are inserted at irregular intervals. Negative leap seconds are rarely used, indeed, to date a negative leap seconds has never been inserted.
Leap seconds are generally inserted either on the last day of June, 6-months into the year, or on the last day of December, at the end of the year. There have been a total of 35 leap seconds introduced since 1972. The last few leap seconds to be inserted were on:
June 30 1997
December 31 1998
December 31 2008
June 30 2012
GPS and Radio Time and Frequency Broadcasts and Leap Seconds
Radio time and frequency broadcasts, such as MSF, DCF77 and WWVB, continuously transmit UTC time information. However, along with each time-stamp, they also transmit a number of flags. One of the flags is generally used to indicate whether a leap second is pending. One hour before a leap second, a flag is set to announce the imminent insertion of a leap second. Similarly, most GPS receivers also provide flags announcing the impending insertion of a leap second. The actual time-stamp provided by both GPS and radio receivers at the leap second will be 23:59:60 on the day of insertion.
NTP and Leap Seconds
Network Time Protocol (NTP) implements algorithms that can receive leap second announcements from GPS or radio reference clocks. The NTP daemon then informs network time clients and it’s host operating system of the impending insertion. Leap second announcements may also be received and acted upon from external NTP time servers.
Andrew Everett has worked in the Computer Time and Frequency sector for almost his entire career. He now leads TimeTools development department. Andrew has written many articles that help IT professionals make informed decisions about network and computer systems timing solutions.