But "clocks" can only be in "sync" – i.e. record the same time between each "tick" – when they are in the same inertial frame – from special relativity. In addi

But "clocks" can only be in "sync" – i.e. record the same time between each "tick" – when they are in the same inertial frame – from special relativity. In addition, there is gravitational time dilation. Thus, the "reality" is that no two clocks are in sync. As Rovelli points out in "Reality Is Not What It Seems", with the precise clocks we have today, it's possible to measure this effect for a difference in altitude of a few centimetres.