Anyone who phones colleagues in New York, calls relatives in Australia or plans a flight to Bangkok asks the same question: what time is it there right now? The answer depends not only on the time zone but also on the date, because many countries change their clocks twice a year, and by no means all on the same day. This guide explains how time zones work, what UTC, CET and CEST stand for and how to work out any time difference yourself.
The short answer
Target time = your time − your UTC offset + the target's UTC offset. Germany uses UTC+1 (CET) in winter and UTC+2 (CEST) in summer. When it is 12:00 in Berlin in December, it is 6:00 in New York (UTC−5) and 20:00 in Tokyo (UTC+9).
The quickest way is the time zone converter: it knows the daylight saving rules of every country for any date and shows whether it is working hours, evening or night at the destination.
Why time zones exist
Until the 19th century every town kept its own local time: noon was when the sun was highest. Because the Earth turns 360 degrees in 24 hours, this noon shifts by 4 minutes per degree of longitude. Between Aachen and Görlitz, at opposite ends of Germany, that is about 36 minutes. As long as people travelled by coach, nobody minded. With the railways the many local times became a problem, because timetables needed a common clock.
In 1884, delegates from 25 countries agreed at the International Meridian Conference in Washington to use the meridian through the observatory at Greenwich near London as the prime meridian. This gave rise to the system of time zones: 360 degrees divided by 24 hours makes 15 degrees per hour. In the German Empire, Central European Time, based on the 15th meridian east that runs through Görlitz, has applied uniformly since 1 April 1893.
UTC, GMT, CET and CEST: the key terms
| Abbreviation | Meaning | Offset |
|---|---|---|
| UTC | Coordinated Universal Time, the reference for all time zones, derived from atomic clocks | ±0 |
| GMT | Greenwich Mean Time, in everyday use the same as UTC; winter time in the UK | ±0 |
| CET | Central European Time, standard time in Germany (German: MEZ) | UTC+1 |
| CEST | Central European Summer Time (German: MESZ) | UTC+2 |
| EET / EEST | Eastern European Time (e.g. Greece, Finland) | UTC+2 / UTC+3 |
| WET / WEST | Western European Time (e.g. Portugal, Canary Islands) | ±0 / UTC+1 |
UTC itself has no daylight saving time, which makes it a neutral reference: every local time can be written as "UTC plus offset". In Germany, the national metrology institute (PTB) in Braunschweig disseminates the legal time, among other ways via the DCF77 longwave transmitter that radio-controlled clocks follow.
Time zones are political, not geometric
In theory there would be 24 zones of 15 degrees each. In practice, countries set their own time, and borders rarely follow lines of longitude. A few examples:
- China spans about 60 degrees of longitude but uses a single time nationwide (UTC+8). In the far west the sun therefore rises only around 10 a.m. in winter.
- Spain lies on the same longitude as Great Britain but uses Central European Time. The Canary Islands, on the other hand, keep British time and are one hour behind Germany.
- India has a single zone with a half hour: UTC+5:30. Nepal is at UTC+5:45, Iran at UTC+3:30.
- Russia spans 11 time zones, and France, with its overseas territories, even 12.
- Samoa switched sides of the international date line in 2011 and simply skipped 30 December 2011.
Today almost 40 different offsets are in use, from UTC−12 to UTC+14. The islands of Kiribati (UTC+14) and uninhabited Baker Island (UTC−12) are 26 hours apart even though both lie in the Pacific: for a few hours each day it is "today", "yesterday" and "tomorrow" at the same moment somewhere on Earth.
Daylight saving time: why the difference changes
Many countries put their clocks forward by one hour in summer. The EU does this jointly: summer time starts on the last Sunday in March and ends on the last Sunday in October, both at 01:00 UTC (Directive 2000/84/EC). In Germany the clock jumps from 2 to 3 a.m. in March and goes back from 3 to 2 a.m. in October. The exact dates until 2030 and the rules are covered in Clock changes in Europe: dates until 2030.
For time differences this means three things:
- Both sides change, on the same day: the difference stays the same. Example London: always 1 hour behind Germany.
- Only Germany changes: the difference changes by one hour twice a year. Thailand is 6 hours ahead of Germany in winter and 5 in summer. The same applies to Japan (8 and 7), China (7 and 6), India (4:30 and 3:30) and Dubai (3 and 2).
- Both change, but on different days: then there are weeks with a different gap. The USA starts summer time on the 2nd Sunday in March and ends it on the 1st Sunday in November. In the weeks in between, New York is only 5 instead of 6 hours behind Germany. More in Time difference Germany–USA.
In the southern hemisphere it is the other way round: summer there is winter here. Sydney puts its clocks forward in early October and back in early April. That is why Sydney is 10 hours ahead of Germany in January but only 8 in July.
Calculating a time difference step by step
The safe route always goes via UTC. Here is how to work out the time in Sydney when you call from Berlin at 20:00 on 15 January 2027:
- Find your own offset on that date: Berlin is on standard time in January, i.e. UTC+1.
- Convert to UTC: 20:00 − 1 hour = 19:00 UTC.
- Find the target's offset on that date: Sydney is on summer time in January, i.e. UTC+11.
- Calculate the target time: 19:00 + 11 hours = 30:00, i.e. 06:00 the next day, Saturday, 16 January.
As a formula:
If the result is more than 24:00, it is already the next day at the destination; if it is less than 0:00, it is still the previous day there. This matters for flights: leave Frankfurt for Asia in the evening and, after about ten hours in the air, you land around noon the next day. Fly to New York in the morning and, after eight and a half hours, you arrive at lunchtime, as if the flight had taken only two and a half hours.
The most common mistakes
- Offset from the wrong date: using the winter difference in July puts you an hour out for Japan, China or Thailand. What counts is always the offset on the day of the appointment.
- Missing the transition weeks: between the 2nd and last Sunday in March and between the last Sunday in October and the 1st Sunday in November, the difference to North America is one hour off. These are exactly the weeks when most phone appointments go wrong.
- Forgetting the date line: a video call on Monday at 9 a.m. in Auckland takes place on Sunday evening in Germany.
- Confusing abbreviations: "CST" can mean Central Standard Time in the USA (UTC−6) or China Standard Time (UTC+8), and "IST" stands for India, Ireland or Israel. City names or UTC offsets are safer.
- The double hour in October: on the night clocks go back, 2:30 a.m. occurs twice. The German Summer Time Ordinance even distinguishes the hours "2A" and "2B".
Time differences to popular destinations
| Destination | Winter (CET) | Summer (CEST) |
|---|---|---|
| London, Lisbon, Canary Islands | −1 h | −1 h |
| Istanbul | +2 h | +1 h |
| Dubai | +3 h | +2 h |
| India | +4:30 h | +3:30 h |
| Thailand | +6 h | +5 h |
| Bali | +7 h | +6 h |
| Japan | +8 h | +7 h |
| Sydney | +10 h | +8 h |
| New York | −6 h | −6 h |
| Los Angeles | −9 h | −9 h |
| Mexico City | −7 h | −8 h |
| São Paulo | −4 h | −5 h |
Winter: as of 15 January 2027, summer: as of 15 July 2027. Mexico largely abolished daylight saving time in 2022, Brazil as early as 2019. The full list of more than 60 cities is on the time zone converter page.
Where the rules come from
Because countries keep changing their time rules, an international community of volunteers maintains the IANA time zone database, also called the tz database. It records every offset and clock change for each region since 1970, many further back. Operating systems, smartphones and browsers use this database, and it is updated several times a year. The time zone converter reads the copy in your browser directly and therefore calculates correctly for past and future dates as long as the rules are known.
Further reading
Conclusion
A time difference is simple arithmetic via UTC, as long as you know the offset on the right date. The pitfalls are daylight saving time, different change dates and the date line. The time zone converter handles all of this for you and shows for every city whether people are working or asleep.
Sources: IANA Time Zone Database (iana.org/time-zones); Directive 2000/84/EC on summer-time arrangements; German Summer Time Ordinance (SoZV) § 2; Physikalisch-Technische Bundesanstalt (PTB), legal time in Germany. As of October 2026.