UTC ⌚️

Coordinated Universal Time or UTC is the primary time standard by which the world regulates clocks and time. It is within about 1 second of mean solar time (such as UT1) at 0° longitude (at the IERS Reference Meridian as the currently used prime meridian) and is not adjusted for daylight saving time. It is effectively a successor to Greenwich Mean Time (GMT).

The coordination of time and frequency transmissions around the world began on 1 January 1960. UTC was first officially adopted as CCIR Recommendation 374, Standard-Frequency and Time-Signal Emissions, in 1963, but the official abbreviation of UTC and the official English name of Coordinated Universal Time (along with the French equivalent) were not adopted until 1967.

The system has been adjusted several times, including a brief period during which the time-coordination radio signals broadcast both UTC and “Stepped Atomic Time (SAT)” before a new UTC was adopted in 1970 and implemented in 1972. This change also adopted leap seconds to simplify future adjustments. This CCIR Recommendation 460 “stated that (a) carrier frequencies and time intervals should be maintained constant and should correspond to the definition of the SI second; (b) step adjustments, when necessary, should be exactly 1 s to maintain approximate agreement with Universal Time (UT); and (c) standard signals should contain information on the difference between UTC and UT.”

A number of proposals have been made to replace UTC with a new system that would eliminate leap seconds. A decision whether to remove them altogether has been deferred until 2023.

世界时 UT 即 格林尼治平太阳时间,是指格林尼治所在地的标准时间,也是表示地球自转速率的一种形式。以地球自转为基础的时间计量系统。地球自转的角度可用地方子午线相对于地球上的基本参考点的运动来度量。为了测量地球自转,人们在地球上选取了两个基本参考点:春分点(见分至点)和平太阳点,由此确定的时间分别称为恒星时和平太阳时。

当需要大于几秒的准确性时,“世界时”是不明确的。由此出现了几个版本的世界时,最常用的是协调世界时间 UTC 和 UT1 。除了 UTC 之外,所有这些版本的UT都基于地球相对于远距离天体(星和类星体)的旋转,但是具有缩放因子和其他调整以使它们更接近太阳时间。UTC 基于国际原子时间,添加闰秒保持在 UT1 的0.9秒内。

国际原子时的准确度为每日数纳秒,而世界时的准确度为每日数毫秒。许多应用部门要求时间系统接近世界时 UT,对于这种情况,一种称为协调世界时的折中时标于1972年面世。为确保协调世界时与世界时相差不会超过0.9秒,在有需要的情况下会在协调世界时内加上正或负闰秒。因此协调世界时与国际原子时之间会出现若干整数秒的差别,两者之差逐年积累,便采用跳秒(闰秒)的方法使协调时与世界时的时刻相接近,其差不超过1s。它既保持时间尺度的均匀性,又能近似地反映地球自转的变化。

按国际无线电咨询委员会(CCIR)通过的关于 UTC 的修正案,从1972年1月1日起 UTC 与 UT1(在 UT 中加入极移改正得到)之间的差值最大可以达到±0.9s。位于巴黎的国际地球自转事务中央局负责决定何时加入闰秒。一般会在每年的6月30日、12月31日的最后一秒进行调整。