Re: Serious question @Stratman
With a simple question you have opened the twin gates of Horology and Metrology. It doesn't matter what color pill you take. I recommend taking them all, and just rolling with it. If anything starts to make sense take more pills and/or turn up your bourbon drip until confusion is restored. Don't worry about the yellow, they don't itch.
In all seriousness, your question is one of the most complex questions any Human has ever asked. Right now, or then, as the case might have been, incredibly intelligent people are engaged in developing proofs and models that have taken years to build all to definitively answer your question and some little smartass post doc has just presented a paper which will distract those incredibly intelligent people just long enough so that he can steal their sanity and ruin the lives of their families.
The sciences of time and measurement are separate entities, but are so intertwined that learning one is learning much of the other, like reading and writing. The stuff of Einstein, Heisenberg and Hawking is a subset of the study of time and measurement. It is really, really deep stuff and much of it is highly theoretical to the point that the correct answer to your question is now incorrect, however that is subject to change if you decide to measure it again and the original answer was correct, but only at that moment but it's on you to determine when that moment was.
The short answer to your question, and I don't think you're going to like it, is that the accuracy of the most accurate clock on Earth is verified to be the most accurate clock on Earth by a group of people who say it is the most accurate clock on Earth. Therefore, it is the most accurate clock on Earth until the aforementioned smartass post doc comes along and proves it isn't.
The Metrologist who runs our tool room is the most intelligent person I've ever met and he's also way up there on the list of the highest paid non-executive employees in the US. He is in charge of calibrating all of our tooling and inspection equipment and is the reason we can create rather large objects with tolerances across any given plane down to a few millionths of an inch. As part of his contract we fully fund his research which is currently an experiment to develop a method to accurately measure the movement of a 1mm tungsten sphere caused by accumulation of individual atoms of the materials that make up the vacuum chambers all these little balls live in. This is the fifth year of that experiment and he is exceedingly pleased that nothing has detectable has occurred. No family or pets. Just a bunch of tungsten balls in oddly shaped sealed containers and that's his thing. But they're all just like that! He goes off to the conferences where NIST type folks like in this story frequent and if the news reported that the entire conference center, after emitting a low humming noise had simply flashed out of existence to the accompaniment of an 19th century accordion ditty I wouldn't be remotely surprised.
All that to say, if you ask him what time it is he responds with 'when?' Just accept they've got a super accurate clock (don't call it a watch :)