Time measurement systems

A clock is an instrument used to measure, keep, and indicate time. The clock is one of the oldest human inventions, meeting the need to measure intervals of time shorter than the natural units: the day, the lunar month, and the year. Devices operating on several physical processes have been used over the millennia.

Watchkeeping or watchstanding is the assignment of sailors to specific roles on a ship to operate it continuously. These assignments, also known as at sea watches are constantly active as they are considered essential to the safe operation of the vessel, and also allow the ship to respond to emergencies and other situations quickly. These watches are divided into work periods to ensure that the roles are always occupied at all times, while those members of the crew who are assigned to a work during a watch are known as watch keepers.

The 12-hour clock is a time convention in which the 24 hours of the day are divided into two periods: a.m. (from Latin ante meridiem, translating to before midday) and p.m. (from Latin post meridiem translating to after midday). Each period consists of 12 hours numbered: 12 (acting as zero), 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, and 11. The 24 hour/day cycle starts at 12 midnight (usually indicated as 12 a.m.), runs through 12 noon (usually indicated as 12 p.m.), and continues just before midnight at the end of the day. The 12-hour clock was developed from the middle of the second millennium BC to the 16th century AD.

The 24-hour clock, popularly referred to in the United States and some other countries as military time, is the convention of timekeeping in which the day runs from midnight to midnight and is divided into 24 hours. This is indicated by the hours passed since midnight, from 0 to 23. This system is the most commonly used time notation in the world today, and is used by the international standard ISO 8601.

An atomic clock is a clock device that uses a hyperfine transition frequency in the microwave, or electron transition frequency in the optical, or ultraviolet region of the electromagnetic spectrum of atoms as a frequency standard for its timekeeping element. Atomic clocks are the most accurate time and frequency standards known, and are used as primary standards for international time distribution services, to control the wave frequency of television broadcasts, and in global navigation satellite systems such as GPS.

A water clock or clepsydra (Greek κλεψύδρα from κλέπτειν kleptein, 'to steal'; ὕδωρ hydor, 'water') is any timepiece by which time is measured by the regulated flow of liquid into (inflow type) or out from (outflow type) a vessel, and where the amount is then measured.

A shot clock is used in basketball to quicken the pace of the game. The shot clock times a play and provides that a team on offense that does not promptly try to score points loses possession of the ball. It is distinct from the game clock, which times the entire game. The shot clock may be referred to by its initial value. For example, in the National Basketball Association (NBA), it may be called the "24-second clock".

ISO 8601 Data elements and interchange formats – Information interchange – Representation of dates and times is an international standard covering the exchange of date- and time-related data. It was issued by the International Organization for Standardization (ISO) and was first published in 1988. The purpose of this standard is to provide an unambiguous and well-defined method of representing dates and times, so as to avoid misinterpretation of numeric representations of dates and times, particularly when data is transferred between countries with different conventions for writing numeric dates and times.

A ship's bell is a bell on a ship that is used for the indication of time as well as other traditional functions. The bell itself is usually made of brass or bronze, and normally has the ship's name engraved or cast on it.

The Catalan time system is the traditional manner in which to tell time in Catalan, and it is exclusive to this language. Telling the time through this system works by dividing it in fractions of a quarter and half a quarter of an hour. Hour-fractions refer to the starting hour, taking into account that when a clock reaches a whole hour (e.g. three o'clock) it actually indicates its end.