This article is a travel topic
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.,:
If you travel has time zone complexities or possible impacts on your health or comfort, consult an expert as you plan it.
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