Time can be thought of as a direction in the 4th dimension, like up and down in 3D (which does not exist on a 2D plane). In this way it corresponds to the "cha

Time can be thought of as a direction in the 4th dimension, like up and down in 3D (which does not exist on a 2D plane). In this way it corresponds to the "change" in the universe. Whenever anything changes, it is moving time forward (and not the other way around). Therefore if we measure some rate of change, and give that rate of change a unit, we have ourselves a way of measuring time. For example, measuring the change in angle for shadows during the day, or measuring the changes in vibration in the Cesium atom. This count then becomes our "time".