How to Calculate Time Zone Differences

Earth rotates 360 degrees in 24 hours, which means each 15 degrees of longitude corresponds to a 1-hour time difference. This is the basis for dividing the world into 24 standard time zones.

UTC (Coordinated Universal Time): The global time standard, based at the Royal Observatory in Greenwich, England. UTC replaced the older term GMT (Greenwich Mean Time) as the official reference because UTC is based on atomic clocks and is more precise.

Time zones are expressed as offsets from UTC. For example, Korea Standard Time (KST) is UTC+9, US Eastern Standard Time (EST) is UTC-5, and India Standard Time (IST) is UTC+5:30.

KST (Korea Standard Time): UTC+9 — South Korea, also matches JST
JST (Japan Standard Time): UTC+9 — Japan
CST (China Standard Time): UTC+8 — All of China, Taiwan, Hong Kong, Singapore
IST (India Standard Time): UTC+5:30 — India (a half-hour offset zone)
CET (Central European Time): UTC+1 — Germany, France, Italy (winter)
CEST (Central European Summer Time): UTC+2 — When daylight saving is active
GMT/UTC: UTC+0 — United Kingdom (winter), Portugal, Iceland
EST (US Eastern): UTC-5 — New York, Washington DC (winter)
EDT (US Eastern Daylight): UTC-4 — When daylight saving is active
PST (US Pacific): UTC-8 — Los Angeles, Seattle (winter)
PDT (US Pacific Daylight): UTC-7 — When daylight saving is active

Daylight Saving Time shifts clocks forward by one hour during warmer months. It typically runs from March to November in the Northern Hemisphere and from October to April in the Southern Hemisphere.

Countries that observe DST: United States, Canada, most of Europe, parts of Australia, and others.
Countries that do NOT observe DST: South Korea, Japan, China, India, most of Southeast Asia.

DST complicates time zone calculations. For example:

• Winter: New York (EST, UTC-5) is 14 hours behind Seoul (KST, UTC+9)
• Summer: New York (EDT, UTC-4) is only 13 hours behind Seoul

Always verify whether the other party's location is currently observing DST when scheduling international meetings.

Scheduling international meetings: Find the overlap in normal business hours (typically 9 AM–6 PM) for all participants. For a Korea–US East Coast call, practical options include 9–10 AM KST (7–8 PM previous day EST) or 10–11 PM KST (8–9 AM EST).

Tracking international shipments: Check whether timestamps in tracking systems use local time or UTC. Most international logistics systems display local times.

Watch for the date line: Time zone differences can push you into a different calendar date. When it's 2 AM Monday in Seoul, it's 12 PM Sunday in New York.

Use Clock-Tani's world clock: Add the cities you need to track on Clock-Tani's world clock to see their current times side-by-side in real time. DST adjustments are applied automatically, eliminating manual calculation errors.

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)

Time zones first became a real problem for me when I started weekly video calls with a client on the US East Coast. 10:00 in Seoul lined up with 20:00 the previous evening in New York, but on the second Sunday of March that suddenly slid to 21:00 because of daylight saving time. I learned that the morning of the first call, the awkward way. Since then I stopped trying to memorize offsets and just pinned Seoul, New York, and London inside Clock-Tani World Clock so I can glance at all three at once before sending any invite.

The worst slip happened with a UK partner. I missed the BST to GMT switch on the last Sunday of October and wrote only "4pm UK time" in an email. They showed up on Zoom an hour early and waited a full half hour before I joined. Since then I always include the UTC offset explicitly, like "16:00 BST (UTC+1)", and right before I send the invite I double-check my laptop clock against an NTP source using the workflow from this related guide on online vs device clock accuracy. It sounds paranoid, but it has saved at least three meetings.

The other lesson is the power of the overlap window. Seoul, New York, and London are only simultaneously awake during a narrow band, roughly 22:00 to 24:00 Seoul time. I now treat that as a hard wall. I once pushed a meeting to 23:00 thinking I could survive on coffee, and it cost me the entire next morning of focused work. The rule now is: pick the overlap, end on time, and never let one call hijack the day after.

Q. What is the practical difference between UTC, GMT, and local times like KST?

UTC is the atomic-clock-based international standard, while GMT is the older mean solar time at Greenwich; in everyday use the two are interchangeable, but formal specifications use UTC. KST is fixed at UTC+9 with no daylight saving. For cross-border invites, writing times in UTC alongside the local zone removes almost all ambiguity, especially during the spring and autumn DST transitions.

Q. When exactly does daylight saving time start and end in the US and Europe?

The US shifts on the second Sunday of March and reverts on the first Sunday of November. The EU shifts on the last Sunday of March and reverts on the last Sunday of October. That leaves a two to three week window each spring and autumn where the offset between, say, New York and London is one hour different from normal, which is when most scheduling mistakes happen.

Q. What are the best meeting windows between Korea and the US or UK?

For New York, 22:00 to 00:00 Seoul time overlaps US morning hours. For London, 18:00 to 21:00 Seoul time works well. For Los Angeles, 07:00 to 09:00 Seoul time catches their late afternoon. Anything outside these bands forces someone into very early morning or late night, which hurts both focus and follow-up.

Q. How should I write times in emails and calendar invites to avoid confusion?

Never write a bare "4pm". Always include both the local zone and the UTC offset, like "16:00 KST (UTC+9)". Better still, send a Google Calendar invite with the timezone explicitly set so each recipient sees their own local time automatically. That single habit eliminates almost every "wait, when is the call?" reply.

Q. Why do some countries use 30 or 45 minute offsets?

India uses UTC+5:30 and Nepal uses UTC+5:45 because of how their reference meridians were chosen historically and how wide their territory is. When scheduling with these regions you have to add or subtract minutes as well as hours, which is easy to miscalculate in your head. Use a world clock tool with named cities rather than mental math.