The tool uses your browser's built-in Intl.DateTimeFormat API with the IANA timezone database (e.g., 'America/New_York', 'Asia/Tokyo') to convert the current UTC time to each city's local time. The IANA timezone database is updated regularly to reflect political changes, such as countries adopting or abolishing daylight saving time.

UTC (Coordinated Universal Time) is the primary time standard used to synchronize clocks worldwide. GMT (Greenwich Mean Time) is a historical timezone based on the meridian at the Royal Observatory in Greenwich, UK. They are effectively the same at whole-hour offsets, but UTC is the modern standard maintained by atomic clocks, while GMT is a geographic timezone.

Daylight Saving Time is the practice of advancing clocks by one hour during summer months to extend evening daylight. The world clock automatically accounts for DST transitions using the IANA timezone database, which knows the exact dates each region switches. This means UTC offsets for cities like New York change between -5 (EST) and -4 (EDT) depending on the season.

The tool supports all major IANA timezone identifiers, covering hundreds of cities worldwide. These include all capital cities and major metropolitan areas across every continent. Some small territories share timezone rules with nearby regions and are listed under the same identifier.

Most timezones are offset from UTC by whole hours, but some countries chose offsets based on their geographic center rather than political convenience. India (UTC+5:30), Iran (UTC+3:30), Nepal (UTC+5:45), and Australia's Chatham Islands (UTC+12:45) are notable examples. These offsets have historical, political, and practical origins specific to each country.