Absolute time

For Newton, absolute time “flows equably without relation to anything external”: one time for the whole universe, the same everywhere and for everyone. Every clock could be set to agree, and “now” is a single flat slice through all of space.

Measured

Optical clocks have measured the slowing of a clock moving at under 10 m/s (2010). Relativity predicts that at 30 m/s a clock runs slow against one at rest by about 5 parts in 10¹⁵, or 0.4 ns a day: for everyday life, Newton’s uniform time is an excellent approximation.

speed 30 m/s

Newton —relativity —

Set the speed of a moving clock. Newton’s clocks still agree; the readout gives what relativity predicts.