The world clock has a built-in set of common cities with their IANA time zone names, and uses the current device Date with Intl.DateTimeFormat to show the time in each place, deriving the current UTC offset and the difference from the local machine from the formatted result. Daylight saving is handled by the browser's own time zone database.

1. 01See the default cities such as Beijing, Tokyo, London and New York.
2. 02Search the built-in cities by local name, English name or IANA time zone and add or remove them.
3. 03Read the local time, the date, the UTC offset and whether it is ahead of or behind your machine.

The UTC offset for America/New_York reflects daylight saving automatically according to the moment of the query and the browser's time zone database, rather than being hard-coded to UTC-5.

• The city list is a fixed set inside the application; arbitrary geographic search is not supported.
• The time source is entirely the local system clock, so a wrong device time skews every city together.
• Time zone rules come from the current browser's or operating system's Intl data, and an old environment may lack the most recent policy changes.
• The day and night icon is only a rough judgement from the local hour being between 6 and 18; it does not compute real sunrise or sunset.

No network access; the list of cities you choose is stored only in the current browser's localStorage.