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Sidereal time ( sy-DEER-ee-əl) is a timekeeping system that astronomers use to locate celestial objects. Using sidereal time, it is possible to easily point a telescope to the proper coordinates in the night sky. In short, sidereal time is a "time scale that is based on Earth's rate of rotation measured relative to the fixed stars".

Viewed from the same location, a star seen at one position in the sky will be seen at the same position on another night at the same sidereal time. This is similar to how the time kept by a sundial can be used to find the location of the Sun. Just as the Sun and Moon appear to rise in the east and set in the west due to the rotation of Earth, so do the stars. Both solar time and sidereal time make use of the regularity of Earth's rotation about its polar axis: solar time following the Sun while, roughly speaking, sidereal time follows the distant fixed stars on the celestial sphere.

More exactly, sidereal time is the angle, measured along the celestial equator, from the observer's meridian to the great circle that passes through the March equinox and both celestial poles, and is usually expressed in hours, minutes, and seconds. Common time on a typical clock measures a slightly longer cycle, accounting not only for Earth's axial rotation but also for Earth's orbit around the Sun.

A sidereal day is approximately 86164.0905 seconds (23 h 56 min 4.0905 s or 23.9344696 h).

(Seconds here follow the SI definition and are not to be confused with ephemeris second.)

The March equinox itself precesses slowly westward relative to the fixed stars, completing one revolution in about 25,800 years, so the misnamed sidereal day ("sidereal" is derived from the Latin sidus meaning "star") is 0.0084 seconds shorter than the stellar day, Earth's period of rotation relative to the fixed stars.The slightly longer "true" sidereal period is measured as the Earth Rotation Angle (ERA), formerly the stellar angle. An increase of 360° in the ERA is a full rotation of the Earth.

Because Earth orbits the Sun once a year, the sidereal time at any given place and time will gain about four minutes against local civil time, every 24 hours, until, after a year has passed, one additional sidereal "day" has elapsed compared to the number of solar days that have gone by.

Linear (or Longitudinal) Timecode (LTC) is an encoding of SMPTE timecode data in an audio signal, as defined in SMPTE 12M specification. The audio signal is commonly recorded on a VTR track or other storage media. The bits are encoded using the biphase mark code (also known as FM): a 0 bit has a single transition at the start of the bit period. A 1 bit has two transitions, at the beginning and middle of the period. This encoding is self-clocking. Each frame is terminated by a 'sync word' which has a special predefined sync relationship with any video or film content.

A special bit in the linear timecode frame, the biphase mark correction bit, ensures that there are an even number of AC transitions in each timecode frame.

The sound of linear timecode is a jarring and distinctive noise and has been used as a sound-effects shorthand to imply telemetry or computers.

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 0), 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 and 11.

The daily cycle starts at 12 midnight (usually indicated as 12:00 a.m.), runs through 12 noon (usually indicated as 12:00 p.m.), and continues until just before midnight at the end of the day. The 12-hour clock was developed from the second millennium BC and reached its modern form in the 16th century AD.

The 12-hour time convention is common in several English-speaking nations and former British colonies, as well as a few other countries. It is an example of a duodecimal system.

Ante Meridian is a misspelling of:

INFO: Full form for CEST is Central European Standard Time in Time category
CEST also has other full forms in other categories mentioned below.

INFO: Full form for CEST is Central Europe Summer Time in Time category
CEST also has other full forms in other categories mentioned below.

INFO: Full form for GET is Greenwich Electronic Time in Time category
GET also has other full forms in other categories mentioned below.

UTC+04:00 is an identifier for a time offset from UTC of +04:00. In ISO 8601 the associated time would be written as 2019-02-07T23:28:34+04:00. This time is used in:

The term ephemeris time (often abbreviated ET) can in principle refer to time in association with any ephemeris (itinerary of the trajectory of an astronomical object). In practice it has been used more specifically to refer to:

Most of the following sections relate to the ephemeris time of the 1952 standard.

An impression has sometimes arisen that ephemeris time was in use from 1900: this probably arose because ET, though proposed and adopted in the period 1948–1952, was defined in detail using formulae that made retrospective use of the epoch date of 1900 January 0 and of Newcomb's Tables of the Sun.

The ephemeris time of the 1952 standard leaves a continuing legacy, through its ephemeris second which became closely duplicated in the length of the current standard SI second (see below: Redefinition of the second).

INFO: Full form for ET is Tiempo Del Este in Time category
ET also has other full forms in other categories mentioned below.

INFO: Full form for ET is Eastern Time [utc - 0500] in Time category
ET also has other full forms in other categories mentioned below.

INFO: Full form for CUT is Incorrect Abbreviation for Coordinated Universal Time (see Utc) in Time category
CUT also has other full forms in other categories mentioned below.

INFO: Full form for ACT is Australian Central Time in Time category
ACT also has other full forms in other categories mentioned below.

INFO: Full form for IST is Irish Summer Time [utc - 0000] in Time category
IST also has other full forms in other categories mentioned below.

Israel Standard Time (IST) (Hebrew: שעון ישראל‎ Sh'on Yisra'el, lit. "Clock of Israel") is the standard time zone in Israel. It is two hours ahead of UTC (UTC+02:00).