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Every time measurement requires the measurement of a change in a physical quantity.

It may be a digital clock number, a pulse, or the Sun's location in the sky.

The concept of time in physics is straightforward: time is a variation or the duration over which changes occur. Unless anything changes, it is hard to tell if time has gone.

The period of time or variation is calibrated by comparing the period of time or variation to a standard. The second, abbreviated s, is the SI unit of time.

An hourglass is a device which contains sand in it. Here are two glasses which are as shown in the figure.

Figure: Hourglass

Here as the change takes place, the sand is falling slowly. So it can be a device to measure time.