UTC Offsets and Time Zone Abbreviations

Questions

What is a UTC offset?

A UTC offset is how far a time zone is ahead of or behind Coordinated Universal Time. UTC+05:30 means local clocks read five and a half hours later than UTC. Offsets are a property of a zone at a point in time, not a fixed label, because many zones change offset for daylight saving.

What is the difference between EST and EDT?

EST is Eastern Standard Time, UTC−05:00, used in winter. EDT is Eastern Daylight Time, UTC−04:00, used from mid-March to early November. Writing 'EST' year-round is a common source of scheduling errors; the IANA name America/New_York covers both and resolves to the correct one automatically.

Why do some zones have a 30 or 45 minute offset?

Offsets do not have to be whole hours. India is UTC+05:30, Nepal is UTC+05:45, parts of Australia are UTC+09:30, and the Chatham Islands are UTC+12:45. These are historical choices about aligning local noon with solar noon, and any tool that assumes whole-hour offsets gets them wrong.

Which time zones do not observe daylight saving?

Most of Asia and Africa, all of India, Japan, China, most of Australia's Queensland, Arizona in the United States, and Saskatchewan in Canada keep one offset year-round. The 'Observes DST' column below shows this for every supported zone by comparing its January and July offsets.