CGPM ends leap seconds in UTC from 20 May 2027
The General Conference on Weights and Measures (CGPM), meeting in Versailles, decided on 15 October 2026 that Coordinated Universal Time (UTC) will run without leap seconds from 20 May 2027. The gap between UTC and Earth-rotation time, UT1, may now grow to 3,600 seconds. A second resolution recommended that a lunar time scale be measured against UTC.
Syllabus: Prelims · Current Affairs & IR; Prelims · Science & Technology; GS3 · Science & Technology
Why this matters for UPSC
GS3 science and GS2 international bodies: Prelims can ask what UTC, TAI and UT1 are, why leap seconds were added, and how the CGPM, BIPM and Metre Convention fit together; Mains, about time synchronisation for critical infrastructure.
Why it matters
Almost every digital system takes its clock from UTC: a bank recording the order of payments, a power grid keeping its generators in step, a mobile network handing a call between towers, a satellite navigation receiver working out a position. Each extra second has forced engineers to patch software, and past ones have disrupted online services. A second taken away has never been tried, so nobody can be sure every system would cope. Ending the adjustments trades a tiny drift between clocks and the Sun for steady, predictable time. Daily life will not notice: time zones and summer-time changes stay as they are. Astronomers and surveyors who need the Earth's exact orientation will instead apply the published correction themselves.
Prelims pointers
- Metre Convention: signed on 20 May 1875 by seventeen nations; it created the BIPM. World Metrology Day, 20 May, marks the signing.
- CGPM voting: each State has one vote; it meets at least once every six years and approves new fundamental metrological determinations.
- UTC(k): national realisations of UTC kept by metrology institutes, whose clock data the BIPM combines to compute UTC.
- SI second: the duration of 9,192,631,770 periods of the radiation from the hyperfine transition of the caesium-133 atom.
- IST: generated at CSIR-NPL, India's national metrology institute, from a bank of caesium clocks and hydrogen masers; India joined the Metre Convention on 11 January 1957.
- Leap seconds: inserted into UTC on the advice of the IERS, so that a minute could last 61 seconds.
Practise a UPSC-style question
With reference to time scales, consider the following statements:
1. International Atomic Time (TAI) is computed from atomic clocks and contains no leap seconds.
2. UT1 is a time scale that follows the angular rotation of the Earth.
3. Coordinated Universal Time (UTC) differs from TAI by a fraction of a second that is adjusted every day.
Which of the statements given above is/are correct?
- (a) 1 and 2 only
- (b) 2 and 3 only
- (c) 1 and 3 only
- (d) 1, 2 and 3
Show the answer
Answer: (a) 1 and 2 only
Answer: (a) 1 and 2 only. TAI is pure atomic time and UT1 is Earth-rotation time; UTC sits a whole number of seconds from TAI.
Statement 1: correct. The BIPM computes TAI as a weighted average of atomic clocks; UTC is obtained from TAI by inserting leap seconds, so TAI itself has none.
Statement 2: correct. The CGPM describes UT1 as the time scale of the Earth's angular rotation, which is not uniform.
Statement 3: incorrect. UTC runs at the same rate as TAI and differs from it only by an integral number of seconds, never by a daily fraction.
Eliminate: Statement 3 is the surest: the UTC-TAI offset is whole seconds. Out: (b), (c), (d).
Trap: Near-miss figure. A fraction adjusted daily sounds technical and plausible, but the real offset changes only in whole seconds.
Remember: TAI = atomic, no leaps; UT1 = Earth's spin; UTC = TAI minus whole seconds.
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