A World Clock displays the current local time across multiple cities and time zones simultaneously. It uses the IANA time zone database (tz database) to accurately calculate each city's local time, accounting for UTC offsets, daylight saving time (DST) rules, and regional timekeeping conventions. By leveraging your browser's built-in Intl.DateTimeFormat API with specific time zone identifiers, the clock stays accurate year-round without manual adjustments.

GMT (Greenwich Mean Time) is a time zone historically based on the prime meridian at Greenwich, London. UTC (Coordinated Universal Time) is the modern atomic time standard that replaced GMT as the global reference. While they are often used interchangeably in casual conversation and have the same offset (UTC+0), UTC is more precise because it is based on atomic clocks rather than astronomical observations. All time zones worldwide are expressed as offsets from UTC, such as UTC+5:30 for India or UTC-5 for US Eastern Time.

Yes! This tool uses your browser's native internationalization engine, which automatically applies the correct daylight saving time rules for each city. When DST begins or ends in a particular region, the displayed time updates accordingly. Cities like New York (Eastern Time) will show EDT in summer and EST in winter, while cities like Tokyo that do not observe DST remain constant year-round. No manual configuration is needed.

There is no hard limit — you can add as many cities as you need. The layout automatically adapts using a responsive CSS grid, displaying cards in 1 to 4 columns depending on your screen width. Your selected cities are saved to your browser's local storage, so they persist even after you close the tab or restart your device. You can add or remove cities at any time with a single click.

While most time zones differ from UTC by whole hours, some regions use fractional offsets for political, geographical, or historical reasons. For example, India uses UTC+5:30, Nepal uses UTC+5:45, and parts of Australia use UTC+10:30. This tool accurately displays these fractional offsets using precise IANA time zone data, so cities like Mumbai or Kathmandu always show the correct local time.

Each city card displays the time difference relative to your local time (e.g., "+3 hours ahead" or "-5 hours behind"), making it easy to find overlapping business hours at a glance. For more detailed planning, you can quickly add all relevant cities and visually scan the grid. The day/night indicator also helps you avoid scheduling meetings during someone's nighttime hours. For precise coordination, look for cities where the day/night badge shows ☀️ (daytime).