This is a list of countries, regions, and territories grouped by time zone.
Although many time zones have descriptive names used by people in them, they are least ambiguously identified by their relationship to UTC (Universal Time, Co-ordinated). UTC used to be called GMT (Greenwich Mean Time), after the Royal Observatory located in the Greenwich area of London.
UTC is also sometimes called Z or Zulu time. A time may be written as e.g. 21:45Z with the Z indicating UTC. The "Z" is for "zero", and "Zulu" is the two-way radio pronunciation of "Z". It comes from the nautical system in which each time zone was assigned a letter.
Time zones east of UTC and west of the International Date Line are specified by the number of hours ahead of UTC (e.g. UTC+4); zones west of UTC and east of the Date Line are specified by the number of hours behind UTC (e.g. UTC-6). Crossing the Date Line going eastward, clocks are turned back a full 24 hours, and vice versa in the opposite direction. (Note: The total span of time zones covers more than 24 hours because the Date Line jogs westward and eastward to keep certain national island groupings on the same calendar day, although they are not within a single time zone.)
Travel Among the Zones
You need to take some care when planning trips that span time zones, e.g.,:
Your "body clock" may experience some stress as you "tell" it to meet business appointments, tours and other obligations perhaps a few or several hours different from the hours you normally rest.
You may miss important obligations simply by not understanding what will be the correct local time as you travel.
Crossing the International Date Line can cause confusion about on what date you'll arrive, e.g.,:
Starting a 12-15 hour flight from the U.S. west coast to Japan or Hong Kong in late evening can land you there in the morning two calendar days later.
If starting the reverse course by midday, you may well arrive but a few clock hours more or less on the same calendar day.
If you travel has time zone complexities or possible impacts on your health or comfort, consult an expert as you plan it.