Time ZonesStandard and daylight names
Where clocks change for summer, the zone has two names. In North America the S becomes a D: Eastern Standard Time (EST) in winter and Eastern Daylight Time (EDT) in summer. People often say "EST" all year, which is wrong by an hour in summer. Writing "ET" or "Eastern Time" avoids the mistake.
Abbreviations with more than one meaning
Abbreviations are not official and several clash:
- IST: India Standard Time (UTC+5:30), Irish Standard Time (UTC+1) and Israel Standard Time (UTC+2).
- CST: Central Standard Time in North America (UTC−6), China Standard Time (UTC+8) and Cuba Standard Time (UTC−5).
- BST: British Summer Time (UTC+1) and Bangladesh Standard Time (UTC+6).
- AST: Atlantic Standard Time (UTC−4) and Arabia Standard Time (UTC+3).
When it matters, give the UTC offset or the city: "3 pm IST (UTC+5:30)" or "3 pm Mumbai time". The converter works with cities, so it never mixes these up.
GMT or UTC?
For everyday use they show the same time. GMT is the name of a time zone, used in the United Kingdom in winter. UTC is the modern scientific standard kept by atomic clocks, and is not a time zone of any country.
How to write a time for people in other countries
A time without a place is only half a message. Three habits remove nearly all confusion. First, name a city, not a set of letters: "4 pm Delhi time" cannot be misread, while "4 pm IST" can. Second, add the UTC offset in brackets when the reader may be anywhere: "4 pm Delhi time (UTC+5:30)". Third, give the date and the day of the week, because the reader may be on the other side of midnight. For a group spread over many countries, give the time once in UTC and let each person convert it. Airlines, shipping and the military do exactly this, and call UTC "Zulu" time, written with a Z after the hour, as in 1030Z.
Why this list is short
Only the abbreviations that people really meet in emails, invitations and broadcasts are listed. The time zone database itself has stopped inventing letters for places that never used any, and shows a plain offset such as +0545 for them. If you meet letters that are not in the table, ask the sender for the city.