The objective referee of a clock is another clock.

Time... you can't see it, you can't hear it, you can't weigh it, you can't measure it in a laboratory. It is a subjective sense of becoming, what we are, in stead of what we were a nanosecond ago, becoming what we will be in another nanosecond. The whole piece of time's a landscape existing, we form behind us and we move, we move through it slice by slice.

The Man from Earth (2007)