https://www.sciencenews.org/article/atomic-clock-general-relativity-time-warp-millimeter-physics

Though we can supply various functional definitions… no one knows what time actually »is. But those of us who think carefully realize a simple fact: it is not like our common definitions at all. In truth, it is ‘passing’ at different rates, and in different ways… all throughout our own body — and there are forms of time we have entirely failed to discern or consider….

https://www.sciencenews.org/article/atomic-clock-general-relativity-time-warp-millimeter-physics