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They could be used to measure the hours even at night, but required manual upkeep to replenish the flow of water.
The Ancient Greeks and the people from Chaldea (southeastern Mesopotamia) regularly maintained timekeeping records as an essential part of their astronomical observations.
The reforms of Julius Caesar in 45 BC put the Roman world on a solar calendar.
During the French Revolution, a new clock and calendar were invented in attempt to de-Christianize time and create a more rational system in order to replace the Gregorian calendar.
Increasingly, personal electronic devices display both calendars and clocks simultaneously.
The number (as on a clock dial or calendar) that marks the occurrence of a specified event as to hour or date is obtained by counting from a fiducial epoch—a central reference point.
Currently, the international unit of time, the second, is defined by measuring the electronic transition frequency of caesium atoms (see below).
Time is also of significant social importance, having economic value ("time is money") as well as personal value, due to an awareness of the limited time in each day and in human life spans.
Other early forms of calendars originated in Mesoamerica, particularly in ancient Mayan civilization.