Time exists as a mathematical quantity (same as space). Time is not a physical quantity in terms that anything depends on it. Nothing in our physical universe depends on time as well as on space (location), as well as on velocity, and on some other "purely mathematical" so to speak quantities.

This "physical non-existence" of such mathematical quantities is called "shift symmetry (of time, of space, of velocity, of phase, etc)" and is expressed by simple equation: F (t)=F (t+t1). It means, that nothing changes if you shift in time (or in space, or in velocity) any physical process - no observable difference whatsoever.

We call this symmetry term the "energy conservation law,” and "momentum conservation law" for space non-existence (shift symmetry), and "special relativity" for velocity non-existence (shift symmetry), “charge conservation” for phase non-existence, etc.)

Because nothing depends on time, there is no absolute time. No time stones, no other marks indicating time. The only way of "measuring" this mathematical quantity is to take any periodic process say, a pendulum, or a string, or a light bouncing between mirrors, or an electron oscillating in an atom, etc - then call the device a "clock device" or simply "clock”, then take TWO measurements of numbers of oscillations say, at two different locations, or at two different gravity environments, or at two different states of motion, etc., then take a RATIO of these two numbers (can't be one number because time is not absolute) and then label this ratio as "relative rate of one time versus another" or "rate of time versus reference clock rate", or "time in conventional units of time" or "accurate time" or simply "time".

Time used to be defined via pendulum, then via quarts crystal oscillations, then via Cs electron oscillation, and soon via H electron oscillation.