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The world · a living book
Keeping Time
How the world defines, builds, and shares a common second.
1 chapterupdated July 2026primary sources in every chapter
The story so far
The second was once tied to the mean solar day; because Earth’s rotation is irregular, the international measurement system now defines it through a fixed frequency of the caesium-133 atom. BIPM · SI Brochure
World time is assembled rather than read from one master clock: national laboratories contribute clock and comparison data, the BIPM computes Coordinated Universal Time, and laboratories maintain local realizations called UTC(k). BIPM · Circular T
UTC has also been kept near the angle of Earth’s rotation by leap seconds. In 2022, the General Conference on Weights and Measures decided that the allowed difference between UTC and rotational time will increase by or before 2035, with the replacement limit and implementation plan still to be agreed. CGPM Resolution 4
Chapter 1 · July 2026
Who decides what time it is?
The SI second is defined by fixing the caesium-133 hyperfine transition frequency at exactly 9,192,631,770 hertz. BIPM · SI Brochure
UTC is produced by the BIPM at the same rate as International Atomic Time (TAI), while differing from TAI by an integer number of seconds. CGPM Resolution 4 It is not a live signal from a single clock: Circular T publishes the differences between UTC and the local UTC(k) realizations maintained by contributing institutes. BIPM · Circular T
In the United States, NIST generates UTC(NIST) from a continuously running ensemble of atomic clocks and calibrates its frequency with primary standards. NIST · Time realization
The seam between atoms and Earth
UT1 describes Earth’s angular rotation. Under the present arrangement, a leap second is applied when the predicted difference between UT1 and UTC approaches 0.9 seconds. CGPM Resolution 4 The IERS announces whether a leap second will occur in Bulletin C. IERS · Bulletin C
The open question
How closely should civil time continue to follow Earth’s rotation?
Keep the close linkLeap seconds let UTC approximate UT1 closely enough for many users of rotational time; ending that close agreement means those users need a separate correction.
NIST · Leap-second FAQ Favor continuityUnpredictable leap seconds can disrupt digital infrastructure, so the 2022 resolution calls for a larger UT1–UTC limit that can keep UTC continuous for at least a century.
CGPM Resolution 4 The decision does not yet fix the new maximum difference. It asks for a proposal and implementation plan, with the change to occur by or before 2035. CGPM Resolution 4
A living book: chapters are dated and grow as the system changes.