The basic unit of time is the second . The second is defined as the duration of 9,192,631,770 periods of the radiation corresponding to the transition between two hyperfine levels of the ground state of cesium 133. This is what God gave us. This works just fine for farmers, but there are two difficulties for civilized man with quartz timepieces.
Local time is a local phenomenon.
Since the day is 24 hours long, the angular velocity of the Sun is 15 o per hour. The Eastern time zone is centered on 75 o west longitude. Noon EST occurs when the fictitious mean Sun crosses the 7515 th degree of longitude. The fictitious mean Sun differs from the true Sun in that it has a constant angular velocity across the sky. The analemma on older globes demonstrates the Equation of Time.