Time Zone Converter

How Time Zones Work

Time zones divide the Earth into 24 regions, each nominally 15° of longitude wide, corresponding to one hour of time difference. The system was standardized in the late 19th century when railroads needed consistent schedules. Before time zones, each city set its own local time based on solar noon. Today, the IANA timezone database (tzdb) maintains the authoritative list of all time zones, including historical changes and daylight saving time rules.

UTC vs GMT

UTC (Coordinated Universal Time) and GMT (Greenwich Mean Time) are often used interchangeably, but they're technically different. GMT is a time zone — the time at the Royal Observatory in Greenwich, London. UTC is a time standard based on atomic clocks. In practice, they're the same to within a second. However, UTC is the correct technical term for the zero-offset reference time used in computing, aviation, and international standards.

Daylight Saving Time

About 70 countries observe DST, shifting clocks forward by one hour in spring and back in autumn. The United States, Canada, most of Europe, and parts of Australia observe DST. Countries near the equator (Nigeria, India, Singapore, most of Africa and Asia) do not. DST creates headaches for software developers: a city's UTC offset changes twice yearly, making hardcoded offsets unreliable. Always use IANA timezone names, not fixed offsets.

Timezone Quirks

Not all timezones are whole-hour offsets. India is UTC+5:30, Nepal is UTC+5:45, and the Chatham Islands (New Zealand) are UTC+12:45. China spans five geographic time zones but uses a single timezone (UTC+8) nationwide. Russia has 11 time zones. Australia's Northern Territory and South Australia have a 30-minute offset. These quirks make timezone handling in software particularly challenging.

Remote Work and Scheduling

With remote work spanning continents, finding meeting times that work for everyone is a common challenge. The "golden hours" — overlap between business hours across timezones — shrink as teams spread further apart. A team spanning New York to Mumbai has about 3 hours of overlap (9:30 AM Mumbai = 11 PM NYC the night before, but practical overlap is ~6:30–9:30 PM IST / 9 AM–12 PM EST). Tools that visualize business hour overlaps help teams schedule effectively.