World Clock

Every clock in the world is set as an offset from Coordinated Universal Time, UTC: Tokyo is UTC+9, New York UTC−5 in winter. Most offsets are whole hours, but not all: India uses UTC+5:30, Nepal UTC+5:45 and the Chatham Islands of New Zealand UTC+12:45 in their winter. Populated places run from UTC−11 in American Samoa to UTC+14 on Kiritimati in Kiribati, so for one hour each day three calendar dates are in use at once.

Which offset a place uses, and when it changes, is decided by its government and changes more often than people expect. The shared record is the IANA time zone database, maintained by volunteers and used by phones, computers and this page. Each zone in it is named after a city, such as America/New_York or Asia/Kolkata, because names like EST or IST are ambiguous. The world clock reads your browser’s copy of that database, so daylight saving time is applied automatically for each city on the right date, and you don’t need to know who changes clocks when. If a government announces a change at short notice, browsers pick it up with their next update to the database, so an out-of-date phone or computer can show the old time for a while.