This is a digest of this.
An SI second is defined in terms of ceasium.
UT1 and UTC track the Earth's rotation.
UTC is a time in which a second is an SI second.
UT1 tries to keep closer to the real rotation.
|UT1–UTC| < 0.9 seconds.
In 1820 UT1 and UTC advanced at the same rate on the average.
The Earth is slowing.
To avoid long term drift UTC jumps back by one second once a quater as needed.
This is a good intro too.
TAI is like UTC but omits leap seconds.
It was the same in 1958.
GPS uses SI seconds and does not leap and is defined relative to UTC.
TAI = GPS + 19 seconds.
Too Much Accuracy?
People are working towards clocks of a precision of 10–18.
The Schwarzschild radius of the Earth is about 1/2 cm.
The radius of the Earth is about 7000 km or about 109 cm.
Two such clocks whose elevations differ by one centimeter will be drift relative to each other at a rate that could be measured at one part in 109.
It would thus seem necessary to know the altitude of such a clock to 0.1 Å (10–11 m).
This sounds much smaller than the size of the clock.
This is a result of being in a strong gravitational field.
Such a clock in orbit has different problems.
Even the mass of the clock is likely to cause terminological difficulties.