What "exist" and "real" mean.
There are two different kind of existence - one is of the mind and one of the universe (here "the" is redundant).
There are process (and only processes) or to be precise sub-processes in the universe, which itself is a one single unfolding process. No more, no less.
Processes do exist and are real. Nothing else is. What we call atoms are processes too.
For the mind, it's own abstractions and generalisations are real and seem to exist the way Nature exist. Mind sees its own creations as if they are real in the universe.
Strangely enough, most of these abstractions and generalisations (however valid and useful) do not exist outside ones own head (and common culture).
The veil of these concepts which distorts our perception of what is real had been recognised by Upanishadic seers and early Buddhists and is called the veil of Maya by them. The term Maya is even more general and signifies a source of delusion or misperceptions.
Mother Nature, for example, does not possess any numbers and does not count. There is no notion of counting at the level of molecular biology (no counters, no overflows). Information is structural, not digital. There is only structural pattern-matching in DNA/RNA/protein machinery.
This is an implicit proof that lots of concepts does not exist universally. Molecular biology uses literally everything what is real and available, and does not use what does not exist.
There is no notion of time, no notion of simultaneity (yes, everything is built as if nothing else exist - just triggers and message-passing (of "concrete" molecular structures).
The fundamental principle is that what seems real and existing for the mind does not necessarily (and usually do not) exist outside of it.
Mixing and matching concepts of the mind with real processes or their attributes is the cause of the fucking mess which is called modern science, which is actually modern astrology and alchemy based on sectarian consensus, like any religious sect, abstract models and simulations.
There are two concepts which does not exist. First one is simple - a number. There are literally no numbers anywhere of any kind. Just none. The other, less obvious but closely related, is so-called time-space (or absolute, "physical" time). It is nowhere to be found and cannot be experimentally demonstrated.
No, human-made clocks does not measure anything. They are man-made devices which trying to count equal intervals of whatever it happens to be. Of course, these devices are subject to gravitational influences, so they became distorted (as processes) and their reading vary depending on physical aspects of reality.
Have you seen any other devices made to support a human made abstractions? Visit any church or monastery, visit pyramids or Maya temples - there are no shortage of such devices in there.
The famous thought experiment by Einstein is logically unsound, or, to be precise, has type errors, mixing concepts of different kind - existing and nonexistent concepts of the mind.
Again, clocks measure nothing. Time is a derived notion. So each clock is completely unrelated to each other, unlike, say, thermometers or other devices built to measure what is real.