[tz] What's "right"?

Guy Harris gharris at sonic.net
Thu Nov 12 22:05:18 UTC 2020




	A practical scale of time for world-wide use has two essential elements: a realization of the unit of time and a continuous temporal reference. The reference used is International Atomic Time (TAI), a time scale calculated at the BIPM using data from some four hundred atomic clocks in over eighty national laboratories.

In that context, what is a "time scale"?  Does it assign an hour/minute/second value to each second?

It then says

	TAI is a uniform and stable scale which does not, therefore, keep in step with the slightly irregular rotation of the Earth. For public and practical purposes it is necessary to have a scale that, in the long term, does. Such a scale is Coordinated Universal Time (UTC), which is identical with TAI except that from time to time a leap second is added to ensure that, when averaged over a year, the Sun crosses the Greenwich meridian at noon UTC to within 0.9 s.

so UTC is also a "time scale".  ITU-R Recommendation TF.460-6:



	A positive leap-second begins at 23h 59m 60s ...

which suggests that *something* is assigning hour/minute/second values to seconds.

It also says

	A positive or negative leap-second should be the last second of a UTC month ...

suggesting that the clock is tied to a calendar in some fashion.

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