Complete guide to timezone terms, abbreviations, and concepts
Master the language of time zones with our comprehensive glossary. From basic concepts like UTC and GMT to advanced terms like leap seconds and atomic time, this guide covers everything you need to know about timezone terminology.
The primary time standard by which the world regulates clocks and time. UTC is the successor to Greenwich Mean Time (GMT) and is used as the reference for all other time zones.
The mean solar time at the Royal Observatory in Greenwich, London. GMT was the world's time standard until 1972, when it was replaced by UTC.
The time in a specific location, determined by the time zone of that area. Local time varies by location and may include daylight saving time adjustments.
A time standard based on Earth's rotation. UTC is a specific form of Universal Time that is more precise and stable.
A region of the Earth that has the same standard time. The world is divided into 24 time zones, each representing one hour of the day.
The difference in hours and minutes between a time zone and UTC. Positive offsets are ahead of UTC, negative offsets are behind UTC.
The time used during the non-daylight saving period of the year. This is the "normal" time for a time zone.
The practice of advancing clocks during warmer months to extend evening daylight. Clocks are typically moved forward by one hour in spring and back in fall.
An imaginary line in the Pacific Ocean, roughly along the 180th meridian, where the date changes. Crossing the line eastward subtracts a day, crossing westward adds a day.
The 0° longitude line that passes through Greenwich, London. It serves as the reference point for all time zones and is the basis for UTC.
The angular distance east or west of the Prime Meridian, measured in degrees. Each 15° of longitude represents approximately one hour of time difference.
Eastern Standard Time (EST) and Eastern Daylight Time (EDT) are used in the eastern United States and Canada. EST is UTC-5, EDT is UTC-4.
Pacific Standard Time (PST) and Pacific Daylight Time (PDT) are used on the US West Coast. PST is UTC-8, PDT is UTC-7.
Greenwich Mean Time (GMT) and British Summer Time (BST) are used in the United Kingdom. GMT is UTC+0, BST is UTC+1.
Central European Time (CET) and Central European Summer Time (CEST) are used in most of Europe. CET is UTC+1, CEST is UTC+2.
India Standard Time is used throughout India and Sri Lanka. IST is UTC+5:30, which is a half-hour offset from UTC.
Australian Eastern Standard Time (AEST) and Australian Eastern Daylight Time (AEDT) are used in eastern Australia. AEST is UTC+10, AEDT is UTC+11.
Military and aviation term for UTC. The letter "Z" is used to indicate UTC time in military and aviation contexts.
Time zones designated by letters (A through Z, excluding J) used by military and aviation. Each letter represents a specific UTC offset.
Time format using hours from 00:00 to 23:59, eliminating AM/PM confusion. Also called military time or international time.
Time zones that use 30-minute offsets from UTC instead of full hours. These are less common but exist in some regions.
An extra second added to UTC to account for the slowing of Earth's rotation. Leap seconds are added irregularly, typically every few years.
Time based on the position of the sun in the sky. Local solar time varies by longitude and is not uniform across a time zone.
Average solar time over a year, accounting for the Earth's elliptical orbit and axial tilt. This provides a more uniform time standard.
Time standard based on the frequency of atomic vibrations, specifically cesium atoms. This is the most accurate time standard available.
