Comment by zokier
> Among other problems, UTC runs slightly slower or faster depending on how far the Earth is from the Sun. UTC does not run uniformly (apart from Earth-at-sealevel), instead the length of 1 second will slightly grow or shrink depending on the current configuration of the Solar system.
That is completely wrong. UTC seconds are exactly SI seconds, which are all the same uniform length (defined by quorum of atomic clocks).
At sea level on Earth, UTC seconds are all exactly the same, yes. That's the definition of UTC.
The trick is, when you're working with things on the scale of the Solar system and larger, it no longer makes sense to assume your frame is "at sea level on Earth." Your relativistic reference frame has shifted, so (thanks Einstein!) time literally changes underneath your feet.
The primary mechanism (but not the only one) is that a clock on Earth will tick slower when Earth is closer to the Sun, due to the effects of gravitational time dilation.[0]
So yes, a clock on Earth always runs at a uniform rate. But because the universe is fundamentally Einsteinian, that still means that eg if you're working with the orbit of Jupiter or a millisecond pulsar, you will see small introduced timing errors if you try to use UTC (or even UT1 which is UTC without leap seconds) instead of TBD.
[0] https://en.wikipedia.org/wiki/Gravitational_time_dilation