Timezone questions answered
Timezone FAQ
Clear answers to the most common questions about UTC, GMT, Daylight Saving Time, city time differences, timezone abbreviations, and international scheduling.
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UTC and GMT
What is UTC?
UTC (Coordinated Universal Time) is the primary global time standard, maintained by atomic clocks. It never observes Daylight Saving Time and is the reference point from which all other timezone offsets are calculated. UTC is the correct choice for server logs, database timestamps, API responses, and any system that needs one stable global reference.
What is GMT?
GMT (Greenwich Mean Time) is UTC+00:00, historically associated with the Royal Observatory in Greenwich, London. In practice, GMT and UTC show the same time, but UTC is the precise technical standard. The United Kingdom uses GMT in winter and BST (British Summer Time, UTC+01:00) in summer. When people say "GMT" casually, they often mean UK local time, which can be wrong during British Summer Time.
What is the difference between UTC and GMT?
UTC is maintained by atomic clocks and is the international technical standard. GMT is a historical timezone at the same offset (UTC+00:00). For technical systems, use UTC. For UK scheduling, use London time — which is GMT in winter and BST (UTC+01:00) in summer. The practical difference is that GMT can shift (the UK observes DST), while UTC never changes.
Is UTC the same as Zulu time?
Yes. Zulu time (Z) is the military and aviation designation for UTC+00:00. When you see a timestamp like "14:30Z", it means 14:30 UTC. The letter Z comes from the NATO phonetic alphabet (Zulu) and refers to the zero-offset timezone.
Current time in cities
What time is it in London right now?
London uses the Europe/London timezone. In winter (GMT, October to March), London is UTC+00:00. In summer (BST, March to October), London is UTC+01:00. The exact current time updates every second on the London city page.
What time is it in New York right now?
New York uses the America/New_York timezone. In winter (EST, November to March), New York is UTC-05:00. In summer (EDT, March to November), New York is UTC-04:00.
What time is it in Tokyo right now?
Tokyo uses the Asia/Tokyo timezone (Japan Standard Time, JST), which is UTC+09:00 year-round. Japan does not observe Daylight Saving Time, so the offset never changes.
What time is it in Dubai right now?
Dubai uses the Asia/Dubai timezone (Gulf Standard Time, GST), which is UTC+04:00 year-round. The UAE does not observe Daylight Saving Time.
What time is it in Sydney right now?
Sydney uses the Australia/Sydney timezone. In winter (AEST, April to October), Sydney is UTC+10:00. In summer (AEDT, October to April), Sydney is UTC+11:00. Note that Australian seasons are reversed — Sydney observes DST during the Northern Hemisphere winter.
Time differences between cities
What is the time difference between London and New York?
London is 5 hours ahead of New York in winter (when New York is on EST) and 4 hours ahead in summer (when New York is on EDT). The difference shifts by 1 hour for a few weeks each spring and autumn because the US and UK change clocks on different dates. Always use a date-aware converter for exact scheduling.
What is the time difference between New York and Los Angeles?
New York is 3 hours ahead of Los Angeles year-round. Both cities observe Daylight Saving Time on the same schedule (EST/PST in winter, EDT/PDT in summer), so the offset is always 3 hours.
What is the time difference between London and Dubai?
Dubai is 4 hours ahead of London year-round when London is on GMT (winter). When London switches to BST (summer, UTC+01:00), Dubai is 3 hours ahead. Dubai does not observe DST, so the difference changes with the UK clock change.
What is the time difference between London and Tokyo?
Tokyo is 9 hours ahead of London in winter (GMT) and 8 hours ahead in summer (BST). Japan does not observe DST, so the difference changes only when the UK changes its clocks.
What is the time difference between New York and Dubai?
Dubai is 9 hours ahead of New York in winter (EST) and 8 hours ahead in summer (EDT). Dubai does not observe DST, so the difference changes only when the US changes its clocks.
Daylight Saving Time
What is Daylight Saving Time?
Daylight Saving Time (DST) is the practice of advancing clocks by one hour during summer months to extend evening daylight. When clocks "spring forward" in spring, one hour is lost. When they "fall back" in autumn, one hour is gained. Not all countries observe DST.
When do clocks change in the US in 2026?
In the United States (most states), clocks spring forward on Sunday, 8 March 2026 at 2:00 AM (moving to 3:00 AM). Clocks fall back on Sunday, 1 November 2026 at 2:00 AM (moving to 1:00 AM). Arizona and Hawaii do not observe DST.
When do clocks change in the UK in 2026?
In the United Kingdom, clocks spring forward on Sunday, 29 March 2026 at 1:00 AM GMT (moving to 2:00 AM BST). Clocks fall back on Sunday, 25 October 2026 at 2:00 AM BST (moving to 1:00 AM GMT).
Which countries do not observe Daylight Saving Time?
Countries that do not observe DST include: Japan, China, India, most of Africa (including Nigeria, Kenya, Egypt, South Africa), UAE, Saudi Arabia, Singapore, most of Southeast Asia (Thailand, Vietnam, Indonesia, Malaysia), Iceland, and others. These countries maintain a fixed UTC offset year-round.
Why does the time difference between the US and UK change in spring?
The US and UK change their clocks on different dates. In 2026, the US springs forward on 8 March and the UK springs forward on 29 March. For the three weeks between these dates, the time difference between New York and London is 4 hours instead of the usual 5. The same pattern happens in autumn when the UK falls back before the US.
Timezone abbreviations
What does EST stand for?
EST stands for Eastern Standard Time. It is UTC-05:00 and is used in eastern North America during winter (roughly November to March). During summer, eastern North America switches to EDT (Eastern Daylight Time, UTC-04:00).
What does BST stand for?
BST stands for British Summer Time. It is UTC+01:00 and is the UK's Daylight Saving Time, active from late March to late October. Outside BST, the UK uses GMT (UTC+00:00). Note: BST can also mean Bangladesh Standard Time (UTC+06:00) in other contexts.
What does CET stand for?
CET stands for Central European Time. It is UTC+01:00 and is used in much of Central Europe during winter. During summer, most CET countries switch to CEST (Central European Summer Time, UTC+02:00). Cities using CET/CEST include Paris, Berlin, Rome, Madrid, Amsterdam, and Warsaw.
Why are timezone abbreviations sometimes confusing?
Timezone abbreviations are not globally unique. CST can mean Central Standard Time (UTC-06:00 in North America), China Standard Time (UTC+08:00), or Cuba Standard Time (UTC-05:00). IST can mean India Standard Time (UTC+05:30), Irish Standard Time (UTC+01:00), or Israel Standard Time (UTC+02:00). For exact scheduling, always use a city name or the full IANA timezone identifier.
Scheduling and meetings
What is the best time to call London from New York?
The best overlap for a call between New York and London is 9 AM–1 PM New York time (2 PM–6 PM London) in winter. In summer, the window is similar but check the exact dates around DST transitions. Use the meeting planner to see the exact overlap for any date.
How do I convert time between two cities?
Use the Timezzon time zone converter. Search for any two cities, optionally pick a specific date, and the converter shows the exact time in both locations with DST rules applied automatically. The 24-hour comparison table highlights business hours in the destination city.
How do I find a meeting time that works for multiple timezones?
Use the Timezzon meeting planner. Add up to 5 cities and the planner shows a 24-hour board with work hours (green), edge hours (amber), and sleep hours (grey) for each location. It automatically highlights the best overlap window and generates a shareable link.
Why does the time difference between two cities change throughout the year?
When one city observes Daylight Saving Time and the other does not — or when they change clocks on different dates — the UTC offset of one city shifts while the other stays the same. This changes the difference between them. For example, London and New York have a 5-hour difference in winter but a 4-hour difference in summer, and a 4-hour difference for a few weeks in spring when the US has changed but the UK has not yet.
Technical questions
What is the IANA timezone database?
The IANA timezone database (also called the tz database or Olson database) is the authoritative global list of timezone rules. Each entry is identified by a region/city name such as Europe/London or America/New_York. These identifiers encode all historical and current DST rules for that location. Timezzon uses this database via the Luxon library — the same database used by most operating systems and browsers.
How accurate is Timezzon's world clock?
Each city clock is anchored to a server-side UTC timestamp rendered at page load. The browser ticks forward using performance.now() drift (not the device clock, which can be wrong) and re-syncs from the server every 5 minutes. Accuracy is typically within 1 second of true time.
What is the difference between a timezone and a UTC offset?
A UTC offset is a fixed number like UTC+05:30. A timezone is a named region with rules that determine the current offset, including DST transitions. For example, the America/New_York timezone has an offset of UTC-05:00 in winter and UTC-04:00 in summer. Using a timezone name is more reliable than a fixed offset because it automatically applies the correct rules for any date.
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