Time provides us with a measure of change by putting dates on moments, fixing the durations of events, and specifying which events happen before which other events. In order to do that, some method of time measurement is needed. The science or art of the accurate measurement of time is known as chronometry (or, less formally, timekeeping). A similar concept, horology, usually refers to mechanical timekeeping devices or timepieces. Time can be measured both in terms of the absolute moment when a particular event occurs, or in terms of a time interval, i.e. the duration of a continued event.
There are two main methods used in the everyday measurement of time, depending on the accuracy required or the interval covered. A clock is a physical mechanism that counts the ongoing passage of time, and is mainly used for more accurate timekeeping and for periods of less than a day. A calendar is a mathematical abstraction used for calculating more extensive periods of time (i.e. longer than a day). Typically, both methods are used together to specify when in time a particular events occurs (e.g. 12:30PM on 16 December 2013). Even before such methods were devised, mankind has always used more informal methods for basic timekeeping, such as the cycle of the seasons, and of day and night, and the position of the Sun in the sky.
Chronology, as opposed to chronometry, is the science of arranging events in their order or sequence of occurrence in time, and is mainly used for studying the past. For convenience, events can be put into chronological groups, a process known as periodization. Chronology, periodization and the interpretation of the past are together known as the study of history.
The measurement of time involves the use of various different units of measurement, depending on the time scales and periods under consideration. These range from the almost infinitesimal units employed in physics, though the everyday units (e.g. seconds, minutes, hours, days, months, years, etc), to the much larger units used in geological and cosmological time scales.
Different time standards, specifications for the measurement of time, have been in use throughout history, although modern globalization and scientific internationalism have led to at the adoption of highly accurate and largely universal standards of time measurement and central reference points.