What is the scientific definition of time?

In physical science, time is defined as a measurement, or as what the clock face reads. With the advent of atomic timekeeping and the International System of Units, time is measured in units of seconds and held to the standards of the UTC time stamp.

Other definitions or descriptions of time include the "arrow of time" in which individuals remember the past but not the future, as well as Einstein's theory of relativity, which measures time as a function of the speed of light. Time has also been described as a subjective measurement that is also a social construction, which means that individuals do not sense time passing unless they measure it using an external device.