Glossary of Time Terms
Coordinated Universal Time (UTC)
The primary time standard by which the world regulates clocks and time. It is a successor to GMT and is based on atomic time. Time zones are expressed as positive or negative offsets from UTC (e.g., UTC-5).
Greenwich Mean Time (GMT)
An older time standard based on the mean solar time at the Royal Observatory in Greenwich, London. While technically a time zone (UTC±00:00), the term is often used interchangeably with UTC in casual contexts.
International Date Line (IDL)
An imaginary line on the surface of the Earth that runs from the north to the south pole and demarcates the change of one calendar day to the next. It roughly follows the 180° longitude meridian.
Daylight Saving Time (DST)
The practice of advancing clocks (typically by one hour) during warmer months so that darkness falls at a later clock time. The intention is to make better use of natural daylight.
Prime Meridian
The line of 0° longitude, used as the origin for measuring longitude east and west. The modern Prime Meridian is located at Greenwich, London.
Leap Second
A one-second adjustment that is occasionally applied to UTC to keep it closely aligned with mean solar time (UT1), which varies due to irregularities in the Earth's rotation.