A combination of this and a particular bug in Linux caused computers to behave erratically and led to failures in some datacentres the last time a leap second was introduced in 2012, notably in one large airline booking system. Any software reading off a clock twice within a second might find the deeply confusing situation of the second time-stamp predating the first. Unfortunately, the way NTP implemented leap seconds in Unix and Linux operating systems (which run most internet servers) made things worse: by leaping back in time to the beginning of the final second and repeating it. There are no numbers left on the scale that could represent the time 23:59:60. Looked at this way, adding one to any time value representing 23:59:59 will always end up with 00:00:00. Instead, it’s more convenient to represent time as a single number, a running count of seconds. This turned out to be impractical, however, as computer software rarely breaks time into separate variables for hours, minutes and seconds. The leap second’s inventors envisaged that a digital clock displaying UTC, which would normally step from 23:59:59 to 00:00:00, would instead insert an additional 61st second, displayed as 23:59:60. And it had become common practice to synchronise computer clocks over the Internet using the Network Time Protocol (NTP), which posed the question of how computers should implement leap seconds. High-precision time broadcasts from the GPS navigation system enabled new safety-critical applications, such as aircraft navigation and control, where time variables immediately affect the trajectory of vehicles. A large effort was underway to tackle the millenium bug in computer systems, which led engineers to start worry about other time-related disruptions. While this worked well, by the late 1990s there were concerns. Leap seconds since 1972 – 25 and counting. Atomic clock signals were soon used to control standard-frequency radio transmitters, which telecommunication engineers could use to calibrate and synchronise equipment. These provide a far more stable and easy-to-use definition of time, based on a particular microwave frequency absorbed by caesium atoms. The varying length of the day has been known for centuries but only became a practical concern (outside astronomy) with the invention of atomic clocks in the 1950s. It is slowing due to the gravitational pull of the moon, with days lengthening by 1.7 milliseconds per century. But this isn’t always the case – June 30 will be a second longer in 2015 with the addition of a leap second, added to reconcile the differences between two definitions of time: one astronomical, the other provided by atomic clocks.īefore the 1950s, time was defined by the position of the sun in the sky, as measured by instruments that monitor the Earth’s rotation. Most people would feel they can count on one day comprising the same number of hours, minutes and seconds as the next.
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