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 across time 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.
Jet lag 
Jet lag is a mismatch between your body clock and the local time wherever you are. It's caused by rapid travel across time zones.
Jet lag is also compounded by the fact that long hours spent on a plane can cause you to sleep too much, or not enough, possibly at the wrong time of day relative to where you departed from. The fatigue from travelling plus the mismatch with local time can leave you ready to fall asleep just after lunch, or being wide awake in the middle of the night, knowing that dawn is still several hours away.
Where flying isn't concerned, it isn't the length of the flight that matters... only the difference in time zones. Flying from Paris to Johannesburg, while it might take you 15–20 hours, wouldn't leave you very jetlagged because there's only 1 hour time difference. A flight from New York to Tokyo, on the other hand, leaves you with a 13-hour difference, which effectively means reversing your sleep/wake schedule.
Flights from east to west, where you gain a few hours, are usually a bit easier, as most people find it easier to stay up a little later than to go to bed earlier. But that only works within about 4 or 5 hours difference.
Recovering from jet lag is a process that, well, takes time. A rule of thumb is that you recover about 1 hour difference per day. You may find that on your way out, you are fine after just a couple of days, but you will really notice the recovery period on your way home, particularly if you didn't stay long enough to fully adjust to the original time difference. At that point your body clock will be really confused and it will take a while for it to sort things out.
You can aid the process a bit by helping to reset your body clock. Sunlight plays a big factor in this and it helps that the sun is out on your first day.
Avoiding jet leg isn't really possible, but you can make things easier on yourself by trying to operate on your new local time as early as possible. If you're going to land early in the day, try to sleep on the plane so you arrive refreshed and ready for a full day of activity. Conversely, if you're going to arrive near the evening, try to stay awake on the plane so that you'll be tired when you arrive and can get a lengthy sleep. As much as possible, spend the daylight hours first few days in your new timezone outdoors.
List of time zones 
- Kiribati Line Islands (except U.S. dependencies - see UTC-11)
- Samoa (during Daylight Saving Time)
- New Zealand: Tokelau
- Kiribati: Phoenix Islands
- New Zealand (during Daylight Saving Time)
- South Pole (during Daylight Saving Time)
- Kiribati: Gilbert Islands
- Marshall Islands
- New Zealand
- Russia: Chukotka, Kamchatka
- South Pole
- Wake Island
- Wallis and Futuna
- Australia: Australian Capital Territory, New South Wales, Tasmania, Victoria (Australian Eastern Daylight Time)
- Federated States of Micronesia: Kosrae, Pohnpei
- New Caledonia
- Russia: Kuril Islands, Magadan Oblast, Sakha
- Solomon Islands
- Australia: Australian Capital Territory, New South Wales, Queensland, Tasmania, Victoria (Australian Eastern Standard Time)
- Federated States of Micronesia: Chuuk, Yap
- Northern Mariana Islands
- Papua New Guinea
- Russia: Primorsky Krai, Khabarovsk Krai, Yakutia, Sakhalin Island
- East Timor
- Indonesia (eastern): Maluku, Western New Guinea
- North Korea
- South Korea
- Russia: Amur Oblast, Zabaykalsky Krai, Yakutia
- Australia: Western Australia (Australian Western Standard Time)
- Hong Kong
- Indonesia (central): (Bali, Borneo)
- Mongolia (most of country)
- Russia: Buryatia, Irkutsk Oblast
- Christmas Island (Australia)
- Indonesia (western): (Java, Sumatra)
- Mongolia (Hovd, Uvs, and Bayan-Ölgii)
- Russia: Kemerovo, Khakassia, Krasnoyarsk Krai, Tomsk, Tuva
- British Indian Ocean Territory (CIA)
- Kazakhstan (Eastern)
- Russia: Altai Krai, Altai Republic, Novosibirsk Oblast, Omsk Oblast, Tomsk Oblast
- British Indian Ocean Territory (NAO)
- French Southern and Antarctic Lands
- Heard Island and McDonald Islands
- Kazakhstan (Western)
- Russia: Astrakhan, Bashkortostan, Chelyabinsk, Kurgan, Orenburg, Perm, Saratov, Sverdlovsk, Tyumen, Ulyanovsk, Volgograd
- Russia: most of European portion, including Moscow, Saint Petersburg, Rostov on Don, Novaya Zemlya, Franz Josef Land, and all railroads throughout Russia including Samara, Udmurtia
- United Arab Emirates
- Russia: Kaliningrad Oblast
- Saudi Arabia
- Democratic Republic of the Congo (eastern)
- Russia: Kaliningrad Oblast
- South Africa
- Bosnia and Herzegovina
- Central African Republic
- Republic of the Congo
- Democratic Republic of the Congo (western)
- Czech Republic
- Equatorial Guinea
- Republic of Macedonia
- San Marino
- Vatican City
- Burkina Faso
- Bouvet Island
- Canary Islands
- Cote d'Ivoire
- Faroe Islands
- Greenland (northeastern)
- Isle of Man
- Northern Ireland
- Saint Helena
- Sierra Leone
- Sao Tome and Principe
- United Kingdom
- Western Sahara
- Fernando de Noronha (Brazil)
- South Georgia and the South Sandwich Islands
- Trindade and Martim Vaz (uninhabited islands) (Brazil)
- Brazil: Brasilia, Rio, São Paulo, Fortaleza, Maceio, Recife, Salvador etc.
- French Guiana
- Greenland (central)
- Saint Pierre and Miquelon
- Antigua and Barbuda
- Brazil: Boa Vista, Campo Grande, Manaus
- Canada ("Atlantic Time"): Labrador, New Brunswick, Nova Scotia, Prince Edward Island, eastern Quebec
- Dominican Republic
- Falkland Islands
- Greenland (west)
- Netherlands Antilles
- Puerto Rico
- Saint Kitts and Nevis
- Saint Lucia
- Saint Vincent and the Grenadines
- Trinidad and Tobago
- U.S. Virgin Islands
- Brazil: Acre
- Canada ("Eastern Time"): Nunavut, most of Ontario, most of Quebec
- Cayman Islands
- Navassa Island
- Turks and Caicos Islands
- United States of America ("Eastern Time"): Maine, New Hampshire, Vermont, New York, Massachusetts, Connecticut, Rhode Island, Michigan except extreme northwestern counties, Indiana except the southwest and northwest corners, Ohio, Pennsylvania, New Jersey, eastern Kentucky, West Virginia, Virginia, Washington, D.C., Maryland, Delaware, eastern Tennessee, North Carolina, Georgia, South Carolina, Florida except western part of panhandle.
- Canada ("Central Time"): Manitoba, Saskatchewan, north-western Ontario
- Costa Rica
- Easter Island
- El Salvador
- Galapagos Islands
- United States of America ("Central Time"): Wisconsin, Illinois, the southwest and northwest corners of Indiana, western Kentucky, western and middle Tennessee, Mississippi, Alabama, Minnesota, Iowa, Missouri, Arkansas, Louisiana, north and east North Dakota, eastern South Dakota, middle and eastern Nebraska, most of Kansas, Oklahoma, most of Texas, part of western Florida(panhandle).
- Canada ("Mountain Time"): Alberta, small eastern portion of British Columbia
- Mexico: Baja California Sur: Chihuahua, Nayarit, Sinaloa, Sonora
- United States of America ("Mountain Time"): southwest North Dakota, western South Dakota, western Nebraska, a sliver of Kansas, Montana, a sliver of Oregon, southern Idaho, Wyoming, Utah, Colorado, Arizona, New Mexico, a corner of Texas
- Canada ("Pacific Time"): most of British Columbia, Yukon
- Clipperton Island
- Mexico: Baja California Norte
- Pitcairn Islands
- United States of America ("Pacific Time"): Washington, northern Idaho, most of Oregon, California, Nevada
- Cook Islands
- French Polynesia: Society Islands, Tuamotu Islands, Austral Islands
- Johnston Atoll
- United States of America: Hawaii, and Aleutian Islands in Alaska