At the largest (cosmological) scale, given a metric such as FLRW, you can define a global family of comoving observers, with a corresponding notion of global ti

At the largest (cosmological) scale, given a metric such as FLRW, you can define a global family of comoving observers, with a corresponding notion of global time: https://en.wikipedia.org/wiki/Cosmic_time https://en.wikipedia.org/wiki/Cosmic_time. This is less applicable at smaller scales, of course, where other solutions of GR (e.g. the Schwarzschild solution) more accurately model the system under study.