And there is also TAI (Pure Earth Atomic Time), and GPST (GPS Time), Galileo Time (GST), BeiDou Time (BDT) — all strongly monotonic sequences of atomic seconds with different frozen offsets. All using weighted averages of different atomic clocks somewhere. Fascinating.

Very interesting to learn that UTC is not purely atomic time, as in sequence of same-lengh seconds defined through atomic vibrations. But starting 2035 UTC will become such strongly monotonic sequence of atomic seconds. There is some beauty in this simplicity.

Also, "which is a weighted average of hundreds of atomic clocks worldwide". Guess even for atomic seconds clocks may go at different speeds due to realitive effects. Really fascinating.