- Both start the new day at Midnight (AM) and all hour numbers are the same until Noon
- At Noon, the 12-Hour clock repeats the same hour numbers but as PM
- At Noon, the 24-Hour clock keeps adding hour numbers: 13 = 1 PM; 14 = 2 PM; 15 = 3 PM; 16 = 4 PM; 17 = 5 PM; 18 = 6 PM; 19 = 7 PM; 20 = 8 PM; 21 = 9 PM; 22 = 10 PM; 23 = 11 PM through 23:59:59 -- then starts over at 00:00:00 (Midnight)
- On both clocks, the minutes and seconds are exactly the same
There are 24 time zones on Earth (each 15 degrees wide x 24 zones = 360 degrees - a full circle) because Earth's rotation requires a 24-hour period. Every minute the Sun is rising somewhere and setting somewhere else; every minute it is precisely noon somewhere at the same time it is precisely midnight somewhere else; every hour it is 1, 2, 3, 4, and 5 AM somewhere and at the same time 1, 2, 3, 4, and 5 PM elsewhere on the globe -- and so on.
Every place EAST of your present location is from 1 millisecond to 12 hours AHEAD (later) by "Sun-Time"; every place WEST of your present location is from 1 millisecond to 12 hours BEHIND (earlier) by "Sun-Time."
Great Britain became the first country to set standard time throughout a region when it established the Greenwich Mean Time standard in the 1840s.
Great Britain, the major world power at that time, placed the center of the first time zone at England's Royal Greenwich Observatory, which was located on the 0-degree longitude meridian. (That line was determined by the Royal Astronomer using a transit telescope.) The international date line was set at the 180-degree longitude meridian in the Pacific Ocean.
As clocks became more accurate and communication became global, there needed to be a starting point from which all other world times were based. Since Great Britain was the world's foremost maritime power when the concept of latitude and longitude came to be, that point for designating longitude was the "prime meridian" which is zero degrees and runs through the Royal Greenwich Observatory, in Greenwich, England, just outside London. Thus, when the concept of time zones was introduced, the "starting" point for calculating all different time zones was/is the Royal Greenwich Observatory.
Prior to, and not that many years ago, (In the US, March 19, 1918 when the national Standard Time Act was passed), most locations observed "Sun-Time" meaning that "official" times varied -- minute-by-minute -- one hour from the eastern to western side of each 15-degree-wide time zone-- making train schelules all but impossible to figure out for long-distance travelers. Jewelers and others set timepieces using the time indicated by a sundial.
NOTE: Two US states do NOT observe Daylight Saving Time: Arizona (except the Navajo Indian Reservation) and Hawaii. Daylight Saving Time also is NOT observed in American Samoa, Guam, Puerto Rico, the US Virgin Islands, and Northern Marianas. Some other major worldwide areas not using DST include (as of January 2013): Russia, Egypt, China. In Australia DST is observed only in New South Wales, Victoria, South Australia, Tasmania, and the Australian Capital Territory.
(NEW day BEGINS -- and -- ENDS here) Eniwetok (Marshall Islands) (GMT minus 12 hours)
Samoa (Polynesian Islands) (GMT minus 11 hours)
Hawaii [No DST] (GMT minus 10 hours)
Alaska Standard Time (GMT minus 9 hours)
Pacific Standard Time (GMT minus 8 hours)
Mountain Standard Time (GMT minus 7 hours)
Central Standard Time (GMT minus 6 hours) (Elbert Wade's Time Zone)
Eastern Standard Time (GMT minus 5 hours)
Atlantic Standard Time (GMT minus 4 hours)
Brazilia Standard Time (Brazil) (GMT minus 3 hours)
Mid-Atlantic Standard (GMT minus 2 hours)
Azores Standard Time (Azores Islands) (GMT minus 1 hour)
Greenwich Mean Time (England) (GMT) [No DST] (also UT/UTC / Zulu (Z) =Military)
Rome Standard Time (Italy) (GMT plus 1 hour)
Israel Standard Time (GMT plus 2 hours)
Moscow Standard Time [No DST] (Russia) (GMT plus 3 hours)
Baku Standard Time (Azerbaijan) (GMT plus 4 hours)
New Delhi (India) Standard Time [NO DST] (GMT plus 5 hours)
Dhakar (Jordan) Standard Time (GMT plus 6 hours)
Bangkok (Thialand) Standard Time (GMT plus 7 hours)
Hong Kong [No DST] (GMT plus 8 hours)
Tokyo (Japan) Standard Time (GMT plus 9 hours)
Sydney (Australia Standard Time) [Varies] (GMT plus 10 hours)
Magadan (Russia Standard Time) [No DST] (GMT plus 11 hours)
Wellington (New Zealand Standard Time) (GMT plus 12 hours)
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