SlothLab ToolsTimezone Converter
Convert time between world cities. Compare multiple timezones at once.
9
06121823
Time
9:00 AM
Seoul, KR (UTC+9)
Target Cities
LondonUTC+0TokyoUTC+9New YorkUTC-5
Converted Time
London, UK
Time
12:00 AM
UTC
UTC+0
Tokyo, JP
Time
9:00 AM
UTC
UTC+9
New York, US
Time
7:00 PM
UTC
UTC-5
Timeline
Seoul9:00 AM
061218
London12:00 AM
061218
Tokyo9:00 AM
061218
New York -17:00 PM
061218
Last Updated: March 16, 2026
How It Works
Timezone conversion works by calculating the difference in UTC offsets between two cities. Each city's local time is expressed as an offset from Coordinated Universal Time (UTC), the global time standard maintained by the International Bureau of Weights and Measures (BIPM) using a network of over 400 atomic clocks worldwide. To convert, subtract the source city's UTC offset and add the target city's offset. For example, converting 9:00 AM Seoul (UTC+9) to London (UTC+0): 9:00 - 9 + 0 = 0:00 (midnight). If the result is negative or exceeds 24, a day change occurs. The calculator also accounts for Daylight Saving Time transitions, which can shift offsets by one hour during summer months in participating regions — affecting approximately 70 countries, primarily in North America, Europe, and parts of Oceania.
Why This Matters
The modern workforce is increasingly distributed across timezones. According to a 2023 Buffer survey, 67% of remote workers regularly collaborate with colleagues in different timezones, and scheduling errors are the number one source of friction in distributed teams. A single missed meeting due to a timezone miscalculation wastes an average of 3.5 person-hours when accounting for rescheduling, follow-up communications, and delayed project timelines.
Timezone complexity extends beyond simple hour differences. Daylight Saving Time transitions create periods where the offset between two cities temporarily changes — for example, the US and Europe switch DST on different dates, creating 2-3 weeks each spring and fall where the normal time difference shifts by an hour. Half-hour and quarter-hour offsets (India at UTC+5:30, Nepal at UTC+5:45, Chatham Islands at UTC+12:45) add another layer of complexity that mental math often gets wrong.
For international businesses, timezone awareness has direct financial implications. Customer support teams need to staff appropriate hours for global coverage. Sales teams need to schedule calls within prospects' business hours. Software deployment teams need to time releases to minimize user impact. This converter provides instant, DST-aware conversions across 24 major world cities, eliminating the mental math errors that lead to missed meetings, delayed launches, and frustrated international partners.
Real-World Examples
Scenario 1 — Distributed engineering standup: A tech company has developers in San Francisco (UTC-8), Berlin (UTC+1), and Bangalore (UTC+5:30). They need a daily standup meeting. The converter reveals that 8:30 AM San Francisco = 5:30 PM Berlin = 10:00 PM Bangalore — not ideal for the India team. Moving to 7:00 AM San Francisco = 4:00 PM Berlin = 8:30 PM Bangalore creates a better compromise, with all team members within reasonable hours.
Scenario 2 — International product launch: A startup is launching a product and wants simultaneous social media posts at 9:00 AM local time in New York, London, and Tokyo. The converter shows that these are 3 separate times: Tokyo 9 AM = New York 7 PM (previous day) = London 12 AM (midnight). They decide on a rolling launch strategy, scheduling separate posts for each market's morning.
Scenario 3 — Travel planning with layovers: A traveler is flying from Sydney (UTC+11) to London (UTC+0) via Dubai (UTC+4). Their flight departs Sydney at 6:00 PM and arrives Dubai at 1:00 AM local time (14 hours later). After a 4-hour layover, they depart Dubai at 5:00 AM and arrive London at 9:30 AM local (7.5 hours later). The converter helps them communicate arrival times to contacts in each city and manage jet lag preparation.
Methodology & Sources
This calculator uses the IANA Time Zone Database (also known as the Olson database or tzdata), the authoritative source for timezone definitions used by virtually all modern operating systems (Linux, macOS, Windows), programming languages (JavaScript, Python, Java), and internet services. The database is maintained by a global community of contributors under the stewardship of ICANN and is updated multiple times per year (typically 5-10 releases) to reflect legislative changes in timezone rules worldwide.
Timezone offsets are calculated relative to Coordinated Universal Time (UTC), which replaced Greenwich Mean Time (GMT) as the international time standard in 1972. UTC is maintained by the Bureau International des Poids et Mesures (BIPM) in Paris, using a weighted average of over 400 atomic clocks in 80+ laboratories worldwide. The resulting time is accurate to within nanoseconds.
The calculator automatically handles Daylight Saving Time (DST) transitions for the approximately 70 countries that observe them. DST rules are complex and change frequently — since 2000, over 30 countries have modified or abolished their DST practices. Notable recent changes include the European Union's ongoing debate to end DST, Turkey's permanent switch to UTC+3 in 2016, and Morocco's adoption of permanent DST (UTC+1) with temporary switches back during Ramadan.
Conversions use JavaScript's built-in Intl.DateTimeFormat API with IANA timezone identifiers (e.g., 'America/New_York', 'Asia/Tokyo'), ensuring accuracy with the browser's timezone data, which is typically updated through operating system patches.
Comparative approaches: Some applications use fixed UTC offsets (e.g., 'UTC+9'), which are simpler but fail to account for DST transitions. Military/NATO time uses single-letter codes (Z for UTC, A-M for east, N-Y for west). The Windows operating system maintains its own timezone database that occasionally diverges from IANA data. This calculator uses the IANA database exclusively as it is the most comprehensive and frequently updated source.
Data sources: IANA Time Zone Database (tzdata), Unicode CLDR (Common Locale Data Repository) for city/region display names.
Limitations: Timezone rules change when governments alter their UTC offset or DST observance, sometimes with very short notice. Recent examples include Samoa skipping December 30, 2011 to jump from UTC-11 to UTC+13, and North Korea adjusting its offset in 2015 and again in 2018. While the IANA database is updated promptly, there may be brief periods where very recent changes are not yet reflected in browser timezone data. Historical conversions for dates before 1970 (when the IANA database begins systematic coverage) may be less accurate. Additionally, sub-second precision is not relevant for this tool's use case of scheduling coordination.
Common Mistakes to Avoid
1. Forgetting Daylight Saving Time transitions: The time difference between New York and London is normally 5 hours, but it becomes 4 hours for about 3 weeks in March (when the US springs forward before Europe) and 6 hours for about 1 week in November (when the US falls back before Europe). Meetings scheduled without checking DST transitions during March-April and October-November frequently result in one party showing up an hour early or late.
2. Assuming all timezones are whole-hour offsets: India (UTC+5:30), Iran (UTC+3:30), Nepal (UTC+5:45), Myanmar (UTC+6:30), and several other regions use half-hour or 45-minute offsets. Rounding these to the nearest hour causes consistent 30-45 minute errors when scheduling with these regions. Always verify the exact offset.
3. Confusing the direction of conversion: When converting from a timezone with a higher offset to a lower one, the local time decreases (and may cross to the previous day), and vice versa. A common error is adding when you should subtract. A reliable method: convert both times to UTC first, then to the target timezone.
4. Not accounting for the International Date Line: When scheduling between East Asia/Oceania and the Americas, the date can change. It is entirely possible for it to be Monday in Tokyo and still Sunday in Los Angeles. Always include the date, not just the time, when communicating across the Pacific.
5. Assuming fixed timezone offsets year-round: Over 70 countries observe DST, and the dates of transition vary. The US, Canada, and Mexico switch in March/November, most of Europe switches in March/October, and Southern Hemisphere countries like Australia and Brazil switch in October/March (reversed seasons). Some countries have recently abolished DST (Turkey, Russia, Argentina), creating historical inconsistencies in recurring event schedules.
Frequently Asked Questions
What is UTC?
UTC (Coordinated Universal Time) is the primary time standard by which the world regulates clocks. It is effectively the successor to Greenwich Mean Time (GMT). All timezone offsets are measured relative to UTC. For example, New York is UTC-5, meaning it is 5 hours behind UTC, while Tokyo is UTC+9, meaning 9 hours ahead.
How do timezone offsets work?
Timezone offsets represent the difference in hours (and sometimes minutes) from UTC. A positive offset (e.g., UTC+9 for Seoul) means the local time is ahead of UTC. A negative offset (e.g., UTC-5 for New York) means it is behind UTC. Some zones have half-hour offsets, such as India (UTC+5:30). To convert between two cities, calculate the difference between their offsets.
What about daylight saving time?
This converter uses standard (non-DST) UTC offsets. During daylight saving time, many regions shift their clocks forward by 1 hour. For example, New York moves from UTC-5 (EST) to UTC-4 (EDT) in summer. If DST is active in either the source or target city, the actual time difference may be 1 hour different from what is shown. Not all countries observe DST — most of Asia, Africa, and South America do not.
Why do some countries have half-hour or quarter-hour timezone offsets?
Most timezones follow whole-hour offsets from UTC, but some countries chose offsets that better align with their solar time. India uses UTC+5:30, Nepal uses UTC+5:45, and Iran uses UTC+3:30. The Chatham Islands (New Zealand) use UTC+12:45. These fractional offsets exist because the countries' geographic positions fall between standard hour offsets, and the governments chose offsets that keep solar noon closer to 12:00 PM for their populations.
How does Daylight Saving Time affect timezone conversions?
DST changes the UTC offset for participating regions, typically by one hour. This means the time difference between two cities can change throughout the year. For example, New York (EST/EDT) and London (GMT/BST) are normally 5 hours apart, but during the periods when one has switched to DST and the other hasn't, the difference can be 4 or 6 hours. This calculator automatically handles DST transitions, but it's important to verify conversions around DST change dates (typically March and November).
How many timezones are there in the world?
There are 38 distinct UTC offsets currently in use, ranging from UTC-12 (Baker Island) to UTC+14 (Line Islands, Kiribati). However, the IANA Time Zone Database tracks over 400 timezone identifiers because many regions share the same offset but have different historical DST rules. Some countries span multiple zones — Russia uses 11, the United States uses 6 (9 including territories), and China technically spans 5 but uses a single timezone (UTC+8) nationwide.
What is the International Date Line and how does it work?
The International Date Line (IDL) runs roughly along the 180° meridian in the Pacific Ocean. When you cross it traveling westward, you advance one calendar day; traveling eastward, you go back one day. The line zigzags to keep island nations and territories on the same date as their trading partners. Notably, Kiribati moved its eastern islands to UTC+14 in 1995 to unify the country on the same business day, making it the first place to enter each new calendar day.
What is the best time to schedule a meeting across US, Europe, and Asia?
There is no single time that falls within standard business hours (9 AM-5 PM) for all three regions simultaneously. The best compromise is typically 8:00-9:00 AM US Eastern (UTC-5), which corresponds to 1:00-2:00 PM in London (UTC+0) and 10:00-11:00 PM in Tokyo (UTC+9). For US-Europe meetings, 8:00-11:00 AM Eastern works well. For Europe-Asia, 8:00-10:00 AM London time overlaps with 4:00-6:00 PM in East Asia.
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