Time Zone Abbreviations

GMT (Greenwich Mean Time) is a time zone based on the sun's position at Greenwich, London. UTC (Coordinated Universal Time) is the modern global reference kept by atomic clocks. In everyday use they both represent offset zero and are often used interchangeably, but UTC is the technically correct term for the international standard. See our UTC/GMT Converter for live conversions.

Abbreviations grew organically within each country before global coordination was common. There is no central registry that assigns unique three-letter codes worldwide, so collisions are inevitable. When you see IST, always check the full name and region — Indian Standard Time (UTC+5:30) and Irish Standard Time (UTC+1) share the same letters but are six and a half hours apart.

EST (Eastern Standard Time) is UTC−5, used in winter in the US and Canadian east. EDT (Eastern Daylight Time) is UTC−4, used during daylight saving from spring through autumn. The same pattern applies across US zones: CST/CDT, MST/MDT, and PST/PDT each shift by one hour when clocks spring forward.

Abbreviations are fine as shorthand among people who share the same region, but for international calls always specify the city and UTC offset — or use our Time Zone Meeting Planner. An email saying “3 pm CST” could mean Chicago, Beijing, or Havana depending on context.

No. Hawaii, Arizona (most areas), Japan, South Korea, China, and many countries near the equator keep the same offset year-round. Others switch between standard and daylight variants seasonally. This reference lists both where they are commonly searched.