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This extra second is displayed on UTC clocks as 23:59:60. Specifically, a positive leap second is inserted between second 23:59:59 of a chosen UTC calendar date (the last day of a month, usually June 30 or December 31) and second 00:00:00 of the following date.

The purpose of a leap second is to compensate for this drift, by occasionally scheduling some UTC days with 86401 or 86399 SI seconds. Therefore, if the UTC day were defined as precisely 86400 SI seconds, the UTC time-of-day would slowly drift apart from that of solar-based standards, such as Greenwich Mean Time (GMT) and its successor UT1. However, the duration of one mean solar day is slightly longer than 24 hours (86400 SI seconds). Like most time standards, UTC defines a grouping of seconds into minutes, hours, days, months, and years. The UTC time standard, which is widely used for international timekeeping and as the reference for civil time in most countries, uses the international system (SI) definition of the second, based on atomic clocks. The most recent leap second was inserted on December 31, 2016 at 23:59:60 UTC. Wikipedia - Coordinated Universal Time Leap Second and AnnouncementĪ leap second is a one-second adjustment that is occasionally applied to Coordinated Universal Time (UTC) in order to keep its time of day close to the mean solar time. In technical contexts, usage of "GMT" is avoided the unambiguous terminology "UTC" or "UT1" is preferred. Saying "GMT" often implies either UTC or UT1 when used within informal or casual contexts. The term Greenwich Mean Time (GMT) does not have a precise definition at the sub-second level, but it is often considered equivalent to UTC or UT1. If high precision is not required, the general term Universal Time (UT) may be used. In the 46 years up to and including 2017, a total of 27 leap seconds have been added the most recent was added on 31 December 2016. Leap seconds keep UTC within 0.9 seconds of UT1. and is based on International Atomic Time (TAI) with leap seconds added at irregular intervals to compensate for the slowing of Earth's rotation. The current version of UTC is defined by International Telecommunications Union Recommendation (ITU-R TF.460-6), Standard-frequency and time-signal emissions. A number of proposals have been made to replace UTC with a new system which would eliminate leap seconds but no consensus has yet been reached. The system was adjusted several times until leap seconds were adopted in 1972 to simplify future adjustments. The UTC was officially formalized in 1963 by the International Radio Consultative Committee in Recommendation 374, having been initiated by several national time laboratories. For most purposes, UTC is synonymous with GMT, but GMT is no longer precisely defined by the scientific community. It is one of several closely related successors to Greenwich Mean Time (GMT). Naval Observatory - Systems of Time UTC ( Coordinated Universal Time)Ĭoordinated Universal Time (UTC) is the primary time standard by which the world regulates clocks and time. The majority of the clocks are caesium clocks the definition of the SI second is written in terms of caesium. Due to the averaging it is far more stable than any clock would be alone (see signal averaging for a discussion). The clocks are compared using GPS signals and two-way satellite time and frequency transfer. TAI as a time scale is a weighted average of the time kept by over 200 atomic clocks in over 50 national laboratories worldwide. TAI in this form was synchronised with Universal Time at the beginning of 1958, and the two have drifted apart ever since, due to the changing motion of the Earth. Specifically, both Julian Dates and the Gregorian calendar are used.

Time coordinates on the TAI scales are conventionally specified using traditional means of specifying days, carried over from non-uniform time standards based on the rotation of the Earth. The 37 seconds result from the initial difference of 10 seconds at the start of 1972, plus 27 leap seconds in UTC since 1972. Since 31 December 2016 when the last leap second was added, TAI has been exactly 37 seconds ahead of UTC. It is the basis for Coordinated Universal Time (UTC), which is used for civil timekeeping all over the Earth's surface, and for Terrestrial Time, which is used for astronomical calculations. International Atomic Time (TAI, from the French name Temps atomique international) is a high-precision atomic coordinate time standard based on the notional passage of proper time on Earth's geoid.