Time zones

See also: Timepieces

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). GMT (Greenwich Mean Time) is nearly synonymous, named after the Royal Observatory located in the Greenwich area of London, but its definitions have varied over time and may vary slightly by context.

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. The total span of time zones covers more than 24 hours because the Date Line jogs westward and eastward to keep certain national island groups on the same calendar day, although they are not within a single time zone.

While when observing "sun time", the sun is at its highest at noon, the political time zones can be stretched out or biased to differ even a few hours from this. Often the only thing that matters is wall clock time, but in areas where you'd prefer indoor activities in the hottest hours, or where daylight hours are few, check times of the hottest hours, the hours with most UV-radiation, or times of sunrise and sunset, depending on your needs. If you don't have these times available, note that the sun moves 15° of longitude by hour, so 30° east of Greenwich, the sun is at its highest at 10:00 UTC; if the time zone is UTC+1, that's 11:00. The hottest time is usually the hours after noon. Darkness comes soon after sunset near the equator, while real dusk comes half an hour after sunset at latitudes of 60°.

Sunrise and sunset can occur at vastly different times than you might be used to, varying from east to west across each time zone. As such, it might be a good idea to check sunrise and sunset times for the time of year you'll be travelling to your destination. As an example: in the American Eastern Time Zone, Boston and parts of Maine and New England, see daybreak around 4:45–5:45AM in the summer and sunset by 8PM, but in the western edge, Indiana Michigan, sunrise isn't until 6:30AM and it isn't dark til 9:15–10:15PM. Another example is Vorkuta in Russia, where sunrise and sunset annually average out at 4:40AM and 4:40PM – with "yesterday" sunrises in May and July. In western Spain, China, and Alaska, daylight averages from 8:30AM to 8:30PM.

You need to take some care when planning trips that cross several time zones, e.g.,

• Your "body clock" may experience some stress as you tell it to meet business appointments, tours and other obligations during hours when you'd be asleep if at home. See Jet lag.
• You may miss an important obligation, or connections with scheduled transport, simply by not understanding what will be the correct local time – or a different time used in your timetable – as you travel.
• Crossing the International Date Line can cause confusion about on what date you'll arrive:
  • 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, in a way, travel back in time, as you land earlier than you started. For example a typical flight from Sydney to LA will take off at lunchtime and land early in the morning on the same calendar date!
• Even just crossing from one time zone to another can be confusing, as you still may arrive before you left, such as when flying from Minsk to Warsaw. You don't even need to fly, or travel east to west. The same happens when taking the north-bound bus from Utsjoki to Tana. Crossing from Kirkenes south to Murmansk Oblast (from November to March), you instead lose two hours, in addition to the travel time.

If your travel has time zone complexities or possible impacts on your health or comfort, consult an expert as you plan it.

If you're in another part of the world, family and friends should be aware of the time zone issue when contacting you. You don't want them calling you in the middle of your night, even if it for them is the time they are used to calling.

Main article: 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, and 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. 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. 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. 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 trying to operate on your new local time as early as possible, and spending the daylight hours in your new time zone outdoors the first few days. 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 can be seen on the map above, some time-zones seem to defy logic and were mostly drawn by national or regional governments to make commerce and administration easier. This can have strange consequences, most notably in the case of China which "should" span five time zones but for political reasons observes the same (Beijing) time in all its territory. To complicate matters, in the restive province of Xinjiang, Beijing time is used by ethnic Han, but UTC +6 is used by ethnic Uyghurs. Departure times of long distance transport are given in one time zone (usually that of the departure point, although the Trans-Siberian railway used to run on Moscow time throughout Russia). On international waters, ship time is decided by the captain (to avoid unnecessary hassle); some Baltic ferries claim to use local time, while what they use is ship time, which corresponds to time at the departure port (with some confusion at arrival). Sir Sandford Fleming, inventor of time zones, was a railway man as rail travel was the advance that made standard time zones necessary. Some railways published schedules with disclaimers like "all times are X time" even before time zones legally existed. Stations sometimes had a clock showing railway time while the town hall or church showed a different "local time" a few minutes off – some stations in Central Europe had a row of clocks for different lines.

The time zones (and the International Date Line) often snake around political boundaries; Chicago lags a full hour behind Thunder Bay because the latter is on Ottawa's side of a provincial boundary. There's a 21-hour time difference between the Diomede Islands, a mere 3 km (1.9 mi) apart but separated by the International Date Line. Visiting the Aroostook Valley Country Club? By the time you get to the 19th hole, you may be wondering where that extra hour went... as the clubhouse was built in New Brunswick as a measure to circumvent Prohibition, putting it an hour ahead of the same course's pro shop in Maine.

Another odd time-zone border lies in Europe where (also mostly due to political reasons) going west from France lets you stay in the same time zone – when you should have to change from Central European time (UTC+1) to UTC or even UTC-1 – but going north from France to Great Britain you will have to change time-zone to GMT (UTC).

A bunch of odd time zones also exist in Australia where the central states and territories of South Australia and the Northern Territory are meant to follow UTC+9, but instead, they follow UTC+9.5 to minimise the difference and disruption when going to the eastern states. Unusually, the outback New South Welsh city of Broken Hill, is completely situated in New South Wales, 30km away from the SA border, follows ACST, and not AEST. And worst of all, there are small towns scattered across the middle of nowhere in WA close to the NT border. The problem here is, that some choose to follow AWST (UTC+8) or ACST (UTC+9.5). There's no discrete border between these time zones, and as such, you could be turning your watches back and forth by 1.5 hours frequently in these areas. And finally, in the Gold Coast-Tweed Heads (Qld-NSW), during summer, you may unexpectedly cross the border, you might have travelled an hour ahead or behind. Daylight Saving Time can further complicate matters.

A handful of time zones differ from UTC not by full hours but by (usually) some other multiple of 15 minutes. North Korea adopted UTC+8:30 briefly (which differs from South Korea's UTC+9 by half an hour) only to drop it just as abruptly during a 2018 thaw in relations; both moves were deemed political by some. Newfoundland island (UTC-3.5, summer UTC-2.5) differs from its neighbours St. Pierre and Miquelon (UTC-3 year-round), Cape Breton (UTC-4, summer UTC-3) and Blanc Sablon (UTC-4 year-round); continue into Labrador and Newfoundland time returns, only to fall back to Cape Breton's time zone somewhere between Red Bay and Cartwright.

Sports fans notice the impact of being in a different time zone any time they plan to view or listen to a live broadcast of a sporting even. For example, someone from the East Coast of the United States who is on the country's West Coast would watch a game three hours earlier per local time than they would while at home. By contrast, Super Bowl Sunday is actually Monday afternoon in Australia.

Global sporting organizations tend to schedule events according to when they would be viewed live in the U.S.A. and Europe. This happened with the 2020 Summer Olympics in Tokyo, when the swimming finals took place in the early morning.