I am aware that ...[this] answer ...is only one among many possibilities. Other authors have argued that the notion of time is irreducible... Until o… - Carlo Rovelli

The thermal time hypothesis. In nature, there is no preferred physical time variable t. There are no equilibrium states ρ₀ preferred a priori. Rather, all variables are equivalent; we can find the system in an arbitrary state ρ; if the system is in a state ρ, then a preferred variable is singled out by the state of the system. This variable is what we call time.

The idea that a well-defined now exists throughout the universe is an illusion, an illegitimate extrapolation of our own experience

[I]n pre-relativistic mechanics, time is a special physical quantity, whose value is measured by physical clocks, that plays the role of the independent variable of physical evolution.