Answers to the questions we are asked most about time zones, offsets and clock changes. Dates and rules are from the IANA time zone database, release 2026c.
What is the difference between a time zone and a UTC offset?
A UTC offset, such as UTC+1, is the difference from UTC at one moment. A time zone, such as Europe/London, is the set of rules that decides which offset applies on each date, including daylight saving changes and their history. London is UTC+0 in winter and UTC+1 in summer, but always in the same time zone. Software should store the zone name, not the offset.
Are GMT and UTC the same?
They show the same clock time, UTC+0, but are defined differently. GMT is an astronomical, historical reference and a time zone name. UTC is the atomic standard every time zone is measured from. Note that UK clocks are on BST (UTC+1) in summer, not GMT. See GMT vs UTC.
How many time zones are there?
It depends what you count. Across January and July 2027, the inhabited zones in the IANA database use 39 different UTC offsets, from UTC−11 to UTC+14. (UTC−12 applies only to uninhabited US islands.) The database's main table has 312 zone entries, because places that share a time today can have different histories. In everyday speech, people usually mean the offsets in use.
Which offsets are not whole hours?
India and Sri Lanka use UTC+5:30, Iran UTC+3:30, Afghanistan UTC+4:30, Myanmar UTC+6:30, Nepal UTC+5:45, and parts of Australia UTC+9:30 (and UTC+10:30 in summer in South Australia). Newfoundland uses UTC−3:30 and the Chatham Islands UTC+12:45. Our Kathmandu and Adelaide pages show two of them.
Which country has the most time zones?
Counting overseas territories, France uses the most offsets, about a dozen, from French Polynesia to New Caledonia. Russia has 11. The United States uses six in its 50 states and several more in its territories. China, despite its width, uses a single official time, UTC+8.
When do the clocks change in 2026 and 2027?
In the UK and the EU: 29 March and 25 October 2026, and 28 March and 31 October 2027. In the US and most of Canada: 8 March and 1 November 2026, and 14 March and 7 November 2027. In south-eastern Australia: 4 October 2026 and 4 April 2027. Full lists are in daylight saving time explained.
Why is the time difference between London and New York sometimes 4 hours?
Because the UK and US change their clocks on different dates. For one week from late October to early November, and for two or three weeks in March, the gap is 4 hours instead of 5. In 2026 that week is 25 October to 1 November. See BST vs EST.
Does all of the US use daylight saving time?
No. Hawaii and most of Arizona do not, nor do Puerto Rico, the US Virgin Islands, Guam, the Northern Mariana Islands or American Samoa. Everywhere else changes on the second Sunday of March and the first Sunday of November.
Which countries changed their time rules recently?
In 2026, British Columbia moved to permanent UTC−7, Alberta to permanent UTC−6 and Morocco to permanent UTC+0. Earlier changes include Mexico ending daylight saving time in most of the country (2022), Egypt bringing it back (2023), Brazil abolishing it (2019) and Turkey staying on UTC+3 all year (2016).
What is the International Date Line?
An imaginary line, mostly following the 180° meridian through the Pacific, where the calendar date changes. Crossing it westwards you skip a day, and crossing eastwards you repeat one. It zigzags around island nations. Kiribati moved it in 1995 so that the whole country shares a date. As a result, its Line Islands use UTC+14, among the first places in the world to start each new day.
What does "Z" or "Zulu" mean after a time?
It means UTC. The letter Z marks the zero-offset zone, and "Zulu" is its radio code word. 1330Z is 13:30 UTC. Aviation, the military and weather services use it so that everyone works to the same clock.
What happens to times during the hour that is skipped or repeated?
When clocks go forward, an hour is skipped: in London, 01:30 on 29 March 2026 never happened. When clocks go back, an hour happens twice: in New York, 01:30 on 1 November 2026 happened first on EDT and again on EST. Our converter warns you about both cases.
Why does my phone show the wrong time in some places?
Phones get their time zone rules from software updates. When a government changes its rules at short notice, as Morocco, British Columbia and Alberta did in 2026, devices that have not been updated may show the old rules. The pages on this site have the rules built in from IANA release 2026c, so they stay correct even on such devices.
Should I write 12-hour or 24-hour times for international readers?
The 24-hour clock is clearer across languages and avoids am/pm mistakes, especially near midnight. Whichever you use, add the city or the UTC offset, and the date if the time is close to midnight in either place.
How accurate are the clocks on this site?
They use your device's clock for the current moment and the IANA time zone rules for the local offset. If your device clock is set correctly, which it usually is when it syncs automatically, the times shown are exact to the second. If your device clock is wrong, our clocks will be wrong by the same amount.
Have a question that is not answered here? Contact us.
Sources
- IANA time zone database: iana.org/time-zones
- GOV.UK, When do the clocks change?: gov.uk
- NIST, Daylight saving time: nist.gov