How to Calculate Time Zone Differences

One hour per 15 degrees of longitude is a starting point for understanding time differences, not a map of exactly 24 fixed civil time zones. Governments set local time rules, including half-hour and quarter-hour offsets and daylight saving. See NIST's time-zone explanation and the IANA time-zone database, which records rules by location and date.

UTC (Coordinated Universal Time): An international reference computed from atomic-clock data contributed by time laboratories. It is not set directly at Greenwich. GMT (Greenwich Mean Time) is a historical name that can still mean UTC+0 in everyday use, but it is not the definition of UTC. See the BIPM explanation of UTC.

For a local-time conversion, check the named city and date as well as its UTC offset. Korea Standard Time (KST) is UTC+9; New York switches between Eastern Standard Time (EST, UTC-5) and Eastern Daylight Time (EDT, UTC-4).

KST (Korea Standard Time): UTC+9 — South Korea
JST (Japan Standard Time): UTC+9 — Japan
UTC+8: China (CST), Taiwan, Hong Kong, and Singapore. They share an offset, not necessarily a local abbreviation.
IST (India Standard Time): UTC+5:30 — India
CET / CEST: UTC+1 / UTC+2 — standard / daylight time in Germany, France, and Italy
GMT / BST: UTC+0 / UTC+1 — standard / summer time in the United Kingdom
EST / EDT: UTC-5 / UTC-4 — standard / daylight time in New York
PST / PDT: UTC-8 / UTC-7 — standard / daylight time in Los Angeles

A city's offset can change with the date. This list does not replace a city-and-date conversion.

Daylight saving advances clocks by one hour for part of the year in some places. Start and end dates vary by jurisdiction and can change. Under current rules, US locations that observe DST switch from the second Sunday in March to the first Sunday in November; EU member states switch from the last Sunday in March to the last Sunday in October. Some US locations do not observe DST. Check NIST's US rules and the Council of the EU's current rules.

For example, Seoul (KST, UTC+9) is 14 hours ahead of New York on dates when New York uses EST (UTC-5), but 13 hours ahead when it uses EDT (UTC-4). Convert the actual meeting date rather than assuming one difference all year.

Scheduling an international meeting: When New York is on EST, 10 PM in Seoul is 8 AM the same day in New York; when it is on EDT, that becomes 9 AM. Likewise, 9 AM in Seoul is 7 PM or 8 PM the previous day in New York. These are conversion examples, not assumed working hours. Check the actual meeting date and named cities in a time-zone-aware calendar.

Tracking international shipments: Check whether the carrier's timestamps use local time or UTC. If the display is unclear, follow the carrier's own explanation.

Watching the date change: At 2 AM Monday in Seoul, it is noon Sunday in New York on EST, or 1 PM Sunday on EDT. Write down the date as well as the time difference.

Using Clock-Tani's world clock: Add cities in the world clock to compare their current local times and offsets side by side. The display follows the current date's time-zone rules. Use a time-zone-aware calendar to convert a future meeting on its actual date.

All differences shown from UTC (Coordinated Universal Time):

Asia:

• Seoul / Tokyo: UTC+9
• Beijing / Hong Kong / Taipei / Singapore: UTC+8
• Bangkok / Hanoi: UTC+7
• New Delhi: UTC+5:30

Europe:

• London: UTC+0 (winter) / UTC+1 (summer)
• Berlin / Paris / Rome: UTC+1 (winter) / UTC+2 (summer)

Americas:

• New York / Toronto: UTC-5 (winter) / UTC-4 (summer)
• Chicago: UTC-6 (winter) / UTC-5 (summer)
• Los Angeles / Seattle: UTC-8 (winter) / UTC-7 (summer)

Oceania:

• Sydney: UTC+11 (summer) / UTC+10 (winter, Australian seasons)

For a meeting involving cities such as Seoul, New York, and London, add them together in Clock-Tani World Clock to compare their current local times. For a future meeting date, use a calendar with time-zone support to check each city's local time. Daylight-saving transitions occur on different dates, so memorizing a permanent offset is unreliable.

Write the date, local time, time-zone name, and UTC offset in the invitation, then verify how the calendar converts it for each participant. Because an offset can change with the date, check recurring meetings again near transitions.

Q. How do UTC, GMT, and KST differ?

UTC is the international reference computed from atomic-clock data. GMT is the historical Greenwich mean-time name, and KST is Korea Standard Time at UTC+9. For a future invitation, include the date, local time, and named location's time zone.

Q. Do the US and EU change clocks on the same dates?

No. Under current rules, US locations that observe DST change on the second Sunday in March and the first Sunday in November. EU member states change on the last Sunday in March and the last Sunday in October. The gap is about two to three weeks in spring and about one week in autumn. Check the rules for the actual year and place.

Q. How do I compare meeting hours in Korea, New York, London, and LA?

At 10 PM in Seoul, it is 8 AM (EST) or 9 AM (EDT) the same day in New York, and 1 PM (GMT) or 2 PM (BST) in London. At 7 AM in Seoul, it is 2 PM (PST) or 3 PM (PDT) the previous day in Los Angeles. Whether a window works depends on participants' real schedules; check each city's date near a DST transition.

Q. How should I write times in email and calendar invites?

Include the date and a named time zone, not just '4 PM'. For example, write '16:00 KST (Asia/Seoul, UTC+9)'. An offset in a daylight-saving region can change with the date, so confirm each attendee's local display in a time-zone-aware calendar.

Q. Why do India and Nepal use half-hour and quarter-hour offsets?

India uses UTC+5:30 and Nepal uses UTC+5:45. A region's civil-time offset is a local choice, not the result of dividing longitude into 15-degree strips. Include minutes in your calculation and verify the named city on the meeting date.