How This Works
The browser ships with the IANA time zone database (also called the tz database or Olson database), which records every named zone and every historical and current daylight-saving rule. Asking the browser to format a date in a given IANA zone returns the correct offset for the chosen instant, including DST transitions.[1]
UTC, GMT, and Local Time
UTC (Coordinated Universal Time) is the modern reference. GMT (Greenwich Mean Time) is for most purposes the same number, though they differ subtly in technical definition. Every local time is UTC plus or minus a fixed offset ??e.g., New York is UTC?? in winter and UTC?? in summer (during EDT). The offset switches when DST starts and ends, which differs by country.
Daylight Saving Time
Most of the United States and most of Europe observe daylight saving time, but on different dates: U.S. DST runs from mid-March to early November; EU DST runs from late March to late October. Most of Asia, sub-Saharan Africa, and many South American countries do not observe DST. Hawaii, most of Arizona, and most U.S. territories also don't.
Picking the Right Zone
IANA zones are usually named "Continent/City" (e.g., America/New_York, Europe/London, Asia/Tokyo) rather than by abbreviation. Three-letter abbreviations like EST/PST/CST are ambiguous (CST means Central Standard Time in the U.S. but China Standard Time in Asia), so this calculator uses IANA names internally and shows a familiar city label on top.
References
[1] IANA Time Zone Database ??iana.org/time-zones