Elapsed time does not change, but the current time is. We measure elapsed time based on a presumably stable measure -- the vibration of cesium under exposure to

Elapsed time does not change, but the current time is. We measure elapsed time based on a presumably stable measure -- the vibration of cesium under exposure to microwaves -- but we want 'now' to be based on the earth's rotation and location with respect to the sun, which is gradually decaying. Atomic vs Astronomical time.