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Greenwich Mean Time (GMT) is the local mean time at the Royal Observatory in Greenwich, London, counted from midnight. At different times in the past, it has been calculated in different ways, including being calculated from noon ; [ 1 ] as a consequence, it cannot be used to specify a particular time unless a context is given.
The time is calculated based on the offset from UTC for the specified timezone taking into account whether daylight saving time is currently active in that timezone. The offset for each implemented timezone is calculated in a sub template (except for UTC).
Time zone abbreviations for both Standard Time and Daylight Saving Time are shown exactly as they appear in the database. See strftime and its "%Z" field. Some of zone records use 3 or 4 letter abbreviations that are tied to physical time zones, others use numeric UTC offsets.
Precise Time and Time Interval (PTTI) is a Department of Defense military and Global Positioning System standard which details a mechanism and waveform for distributing highly accurate timing information. [1] [2] It is similar to pulse per second (PPS) because it indicates the start of each second using a pulse.
To determine "longitude by chronometer," a navigator requires a chronometer set to the local time at the Prime Meridian. Local time at the Prime Meridian has historically been called Greenwich Mean Time (GMT), but now, due to international sensitivities, has been renamed as Coordinated Universal Time (UTC), and is known colloquially as "zulu time".
The time server maintains its clock by using a radio clock or other accurate time source, then all other computers in the system stay synchronized with it. A time client will maintain its clock by making a procedure call to the time server. Variations of this algorithm make more precise time calculations by factoring in network radio ...
An ephemeris is a table of the calculated positions of astronomical objects and various other data, usually for a specific time of the day, either noon or midnight. A uniform time measurement is needed to establish accuracy, and ephemerides will use variously Greenwich Mean Time, Universal Time or Ephemeris Time.
Pips were also heard on many BBC Local Radio stations until the introduction of a new presentation package in 2020. A rare quarter-hour Greenwich Time Signal was heard at 05:15 weekdays on Wally Webb's programme on local radio in the east of England until it ended in March 2020, as part of his "synchronised cup of tea" feature.