In his youth Albert Einstein spent a year loafing aim- lessly. You don’t get anywhere by not ‘wasting’ time – something, unfortunately, which the par… - Carlo Rovelli

While non-relativistic time is the observable quantity measured (or approximated) by physical clocks, in general relativity clocks measure s along their worldline, not t. The relativistic... t is a freely chosen label with no direct physical interpretation. ...The physical content of a solution of Einstein's equations is not in its dependence on t, but ...in what remains once the dependence on t (and x) has been factored away.

I am aware that ...[this] answer ...is only one among many possibilities. Other authors have argued that the notion of time is irreducible... Until our theoretical and experimental investigations tell us otherwise... what is important is to put the alternatives clearly on the table...

Quite remarkably, this was understood before having good clocks... It is one of great intuitions by Einstein. In 1915 he completed the theory of relativity... At the time clocks were not good enough... it was a theory, a good speculation... Today it is not a theory... It's a fact that we measure in the labaratory. ...It's so clear and definite that technology needs this to be taken into account.