The short version of how the world keeps time, and why your friend in Phoenix is sometimes on Pacific time and sometimes isn't.
How time zones work
The Earth turns 15 degrees every hour, so noon — the moment the sun is highest — arrives at a different instant for every longitude. Before railroads, every town simply set its clocks by its own noon. Trains running on dozens of slightly different local times made schedules a mess, so in the 1880s countries began agreeing on standard zones, each roughly an hour wide.
In practice the borders follow politics more than longitude. China spans five geographic zones but uses one clock. India runs on a half-hour offset, and Nepal on a quarter-hour one.
UTC and GMT
Coordinated Universal Time (UTC) is the reference every other zone is measured against. Eastern Time is written UTC−5 in winter, for example, and Tokyo is UTC+9 all year.
Greenwich Mean Time (GMT) is the older name for the time at the Royal Observatory in London. For everyday purposes it matches UTC, but GMT is a time zone used by people, while UTC is a precise scientific standard kept by atomic clocks. The UK uses GMT in winter and British Summer Time (UTC+1) in summer.
Daylight saving time
Many countries move their clocks forward an hour in spring and back in autumn to give evenings more daylight. Not everyone does it, and those who do don't switch on the same day. The US changes on the second Sunday in March and the first Sunday in November; most of Europe changes on the last Sunday of March and October.
That mismatch means the gap between New York and London is four hours for a few weeks each year instead of the usual five. Arizona (outside the Navajo Nation) and Hawaii skip daylight saving entirely, and most of the Southern Hemisphere shifts in the opposite direction. The clocks on this page account for all of it automatically.
Finding a meeting time across zones
Start with the person furthest east and the person furthest west. If the gap between them is eight hours or less, there's usually a window where everyone is inside the workday. Past that, someone takes an early or late call — rotating who does is the fair way to handle a recurring meeting.
Use the converter above: set a time in one city and look for the row of green "Work hours" tags. When you send the invite, include the time in at least two zones so nobody has to do the math.
Common questions
Why is my time shown correctly without asking for my location?
Your device already knows its time zone. The site reads that setting from your browser — it never asks for or stores your physical location.
How many time zones are there?
There are 24 one-hour offsets in principle, but around 38 distinct local offsets are in use today once half-hour and quarter-hour zones are counted, and hundreds of named zone records once each region's daylight saving history is included.
What's the difference between EST and EDT?
EST (Eastern Standard Time) is UTC−5 and applies in winter. EDT (Eastern Daylight Time) is UTC−4 and applies from March to November. Saying "Eastern Time" or "ET" covers both, which is why it's the safer choice in invitations.
Which place is first to see the new year?
Kiribati's Line Islands, at UTC+14, ring in each new day before anywhere else. American Samoa and Baker Island, at UTC−11 and UTC−12, are among the last.