What is a time zone?

A time zone is a region that keeps the same clock time. Each one is a number of hours ahead of or behind UTC, Coordinated Universal Time, the time standard clocks around the world are set by: India is UTC+5:30 and Japan UTC+9. Many places move their clocks an hour forward in summer, for daylight saving time, so their offset changes with the season: Tallinn is UTC+2 in winter and UTC+3 in summer.

The tz database, also called the IANA time zone database, records every time zone's offsets and clock changes, past and present. It names each zone after its region and largest city, such as Europe/Tallinn, America/New_York or Asia/Kolkata. Computers and phones use it to show the right local time.

Tool description

This tool converts a date and time from one time zone to another, for UTC and more than 500 time zones of the tz database. The menus list them by city, such as Tallinn, New York or Kolkata, with the time zone's ID, such as Europe/Tallinn, next to the name. Every time zone converter here works the same way, from the general Timezone Converter to the ones for two major cities, such as Tallinn to New York or London to Tokyo: it starts on the time zones in its name, and the Source and Target menus switch to any other one. Type in a menu to find a time zone by its city or ID.

The converter starts on the current time in the source time zone, so the result shows what time it is in the other one right now. Pick any other date and time and the result changes as you go. The result shows the date as well as the time, so you can see when a time falls on the day before or after. The swap button turns the conversion around.

Daylight saving time

Each time zone converts with the offset it has on the date you pick, so daylight saving time is taken into account. Countries don't all change their clocks on the same day: the US and Canada move them forward on the second Sunday in March and the UK and the European Union on the last Sunday in March, so for two or three weeks New York is only 4 hours behind London instead of 5. In the southern hemisphere, summer time runs from about October to April.

When the clocks go forward and skip an hour, a time inside it is taken as that long after the change: on the night New York's clocks jump from 2:00 to 3:00, 2:30 becomes 3:30. When they go back and show an hour twice, a time in that hour is taken as the first one.

Examples

Time Converted
12:00 in Tallinn on 15 January 10:00 UTC
12:00 in Tallinn on 15 July 9:00 UTC
9:00 in New York on 15 January 14:00 in London
9:00 in New York on 20 March 13:00 in London
12:00 UTC 17:30 in Kolkata
20:00 in Sydney on 15 January 9:00 in London
18:00 in Los Angeles on 15 July 10:00 in Tokyo the next day

How it works

The conversions are done by moment-timezone, a JavaScript library with the tz database, which loads in your browser with the first conversion. The date and time you pick is read on the source time zone's clock, with the offset that zone had then, and shown on the target's clock with its offset at that moment. Everything runs in your browser.

Tips

  • To see what time it is somewhere right now, just open the converter for its time zone: it starts on the current time.
  • Only major cities have converters of their own. Every other time zone, such as Riga or Vancouver, is in the menus of all of them.
  • To find a meeting time that suits several time zones, use the Timezone Meeting Planner.