Coordinated Universal Time (UTC)
The primary time standard by which the world regulates clocks and time. UTC is maintained by atomic clocks and does not observe Daylight Saving Time.
Time Zone Terminology & Definitions
Time Zone Glossary
An authoritative dictionary defining essential global time standards, time zone abbreviations, longitudinal meridians, and software database protocols.
Complete Time Zone Terms Dictionary
Greenwich Mean Time (GMT)
Solar time at the Royal Observatory in Greenwich, London. GMT operates at UTC+0 and serves as the historical baseline for global timekeeping.
Eastern Standard Time (EST)
Standard time zone observing UTC-5 across eastern North America during winter months (New York, Toronto, Miami).
Eastern Daylight Time (EDT)
Daylight saving time zone observing UTC-4 across eastern North America from mid-March to early November.
Central Standard Time (CST)
Standard time zone observing UTC-6 across central North America (Chicago, Dallas, Winnipeg).
Central Daylight Time (CDT)
Daylight saving time zone observing UTC-5 across central North America in summer.
Pacific Standard Time (PST)
Standard time zone observing UTC-8 across western North America (Los Angeles, Seattle, Vancouver).
Pacific Daylight Time (PDT)
Daylight saving time zone observing UTC-7 across western North America in summer.
Mountain Standard Time (MST)
Standard time zone observing UTC-7 across the Rocky Mountain region (Denver, Phoenix, Salt Lake City).
Mountain Daylight Time (MDT)
Daylight saving time zone observing UTC-6 across mountain states in summer.
IANA Time Zone Database
The official international software database (Olson DB) cataloging historical and present time zone rules, daylight saving shifts, and boundary polygons worldwide.
Daylight Saving Time (DST)
The practice of advancing clocks forward by 1 hour during warmer months to extend evening daylight.
Prime Meridian
The 0° longitudinal line passing through Greenwich, England, dividing Earth into Eastern and Western Hemispheres.
ISO 8601
The international standard format for representing dates and times (e.g., 2026-08-06T12:00:00-05:00).
Unix Timestamp
The number of non-leap seconds elapsed since January 1, 1970 00:00:00 UTC, used universally in computer science.
Explore Time Zone Resources
Frequently Asked Questions
What is the difference between UTC and GMT?
UTC is a precise atomic time standard, while GMT is a solar time zone. In practice, both share the UTC+0 offset, but UTC is used in technical computing while GMT is used in geography and law.
Why is the IANA database important?
Operating systems, web servers, and modern browsers rely on the IANA Time Zone Database (America/New_York, America/Chicago) to ensure automatic adjustments for daylight saving transitions.