A clock or chronometer is a device that measures and displays time. The clock is one of the oldest human inventions, meeting the need to measure intervals of time shorter than the natural units such as the day, the lunar month, and the year. Devices operating on several physical processes have been used over the millennia.
An analog pendulum clock made around 18th century
Casio F-91W digital watch, one of the most popular watches ever
Simple horizontal sundial
The flow of sand in an hourglass can be used to keep track of elapsed time.
Time is the continued sequence of existence and events that occurs in an apparently irreversible succession from the past, through the present, and into the future. It is a component quantity of various measurements used to sequence events, to compare the duration of events or the intervals between them, and to quantify rates of change of quantities in material reality or in the conscious experience. Time is often referred to as a fourth dimension, along with three spatial dimensions.
The flow of sand in an hourglass can be used to measure the passage of time. It also concretely represents the present as being between the past and the future.
Horizontal sundial in Canberra
24-hour clock face in Florence
A contemporary quartz watch, 2007