No. But yes. But no. It’s complicated, let me explain.
Greenwich Mean Time (GMT) is the local mean time in Greenwich, London. Local indicates it tracks the position of the Sun on the sky.1 However, because Earth’s rotation speed varies, a second according to GMT has different lengths on different days.
Meanwhile, Coordinated Universal Time (UTC2) is based on atomic clocks which guarantee constant length of a second. Unfortunately, Earth refuses to conform to human standards. To account for that, leap seconds are occasionally applied to UTC. As necessary, one second can be added (resulting in time 23:59:60) or removed (resulting in day ending at 23:59:58) at the end of June or December.3
While GMT and UTC use different methods for tracking time and adjusting to the Earth’s irregular rotation, they are synchronised to within 0.9 s and for everyday purposes they are the same.
Unfortunately things can get more convoluted. Someone may incorrectly use GMT to refer to time zone in London which is UTC+1 during daylight saving time. Furthermore, throughout history there were additional conflicting definitions of GMT.
Conclusion
All in all, it’s best to use UTC to avoid ambiguity. | GMT | UTC |
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| Basis | Based on observation of the stars. | Tied to the time measured by precise atomic clocks. |
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| Stability | A ‘GMT second’ changes as the Earth’s rotation fluctuates. | Every second is of an identical, SI-defined duration. |
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| Adjustments | None necessary. Variable second length handles adjustments. | Leap seconds introduced to stay within 0.9 s of Earth’s rotation. |
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| Ambiguous? | May mean slightly different things depending on situation. | Precisely defined with a singular meaning. |
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Further reading
To anyone interested in the topic I can safely recommend Dennis D. McCarthy and Kenneth P. Seidelmann’s 2009 book ‘TIME — From Earth Rotation to Atomic Physics.’1 The longer explanation is that what’s observed is position of various stars on the sky. Furthermore, due to Earth’s orbit eccentricity the length of days varies throughout the year. The mean time uses observes fictitious mean Sun, constructed through a mathematical model, to average those differences. As a result, Sun will probably not be in the zenith at noon. ↩
2 Yes, the order of letters in the acronym does not match its English expansion. It was chosen to avoid confusion between languages (e.g. CUT in English vs TUC in French for ‘temps universel coordonné’). ↩
3 Even though the adjustment can result in both adding and removing seconds, so far seconds have only been added. Specifically, 27 times since 1972. On that note, in 2022 General Conference on Weights and Measures (CGPM) decided to phase out leap seconds by 2035, so there may not be many more new ones. ↩