Canadian Sandford Fleming invented Standard Time in 1879. He proposed that the world should be divided into 24 equal Time Zones, and that the time within each zone would be the same. Up until then, it had varied from community to community. The railways were the first to adopt the idea and by 1890 all countries had adopted the Standard Time system. Prior to that the U.S. had about 300 local times. Sanford Fleming was knighted in 1897 for his idea.

Each Time Zone contains 15 degrees of longitude. The first time zone is located at Greenwich in England.

There are irregularities caused by the demands of geography and the desire to keep a constant time within some political boundaries. For example, Newfoundland is split down the middle by a time zone boundary. In typical Newfie fashion they decided to split the difference - thus it's always half an hour later in Newfoundland as compared to atlantic standard time.

Time zones are a far worse mess than immediately meets the eye. Fifteen degrees of longitude would be nice, but as the fearsome Lord Brawl observes, the zones themselves are gerrymandered everywhere in order to conform to national and provincial borders, sea coasts, etc. But that's no big deal.

How do you define a time zone? First, there's the offset from UTC (Universal Coordinated Time, rearranged for French grammar). The offsets range from -11:00 hours for Samoa, all the way around past the International Date Line to +14:00 hours for the tiny Pacific island of Kiritimati (the artist formerly known as Christmas Island), east of Samoa. Then you've got the oddballs like Newfoundland at UTC-3:30, and the innovative Chatham Island (New Zealand) at UTC+12:45. Oh, boy!

But that's the easy part.

Where it turns into a genuinely weird mare's nest, is when you throw Daylight Saving Time into the mix. The US state of Indiana is at UTC-5:00, US Eastern Standard Time (EST) -- but Indiana doesn't observe Daylight Saving Time1. Half the year, they're on the East coast; the other half, they're in the Midwest. If you're writing software to handle time, the best way to deal with mutants like Indiana is to define a time zone as the aggregate of the values of all the properties a given area has: UTC offset, DST offset (most of them are an hour, but some are half an hour or whatever), DST start date and start time (not all start at 2:00 AM local time), and DST end date and end time. Since winter and summer in the northern hemisphere are the reverse of those in the southern hemisphere, DST down there starts around the time it ends up here, and vice versa: Quito (Equador) and Baltimore (USA) are at about the same longitude, but their DST adjustments are totally incompatible (for one thing, as Linca rightly observes, Ecuador is on the equator (hence the name) so there's very little sense in observing daylight savings time there anyway -- and they don't. So I'll have to dig up someplace else on Baltimore's longitude but far enough south to do DST).

Thanks to DST, A "time zone" cannot be defined as "all locations which share the same base offset from UTC". Rather, a "time zone" is best defined as "the set of all locations guaranteed to share the same UTC offset at all times". If some little village in Vermont decides to start Daylight Saving Time five minutes earlier than the rest of the state, that guarantee is broken for Vermont w/r/t that little village, and the village has just become its own time zone for all practical purposes. And practical purposes are the only ones that interest me when I've got a ship date breathing down my neck.

The standard library that ships with your operating system almost certainly doesn't have the complete picture; that which ships with Windows is okay for the most part, but don't trust your laptop clock if you've got any important meetings on Kwajalein. You'll show up a day early. Furthermore, rarely does a year go by when somebody, somewhere, doesn't enact a new law and render your information obsolete.

Hooray for Red China: The whole damn country is UTC+8:00, with no DST anywhere.

Going by the pragmatic definition outlined above, there are roughly five hundred and twenty-nine (529) time zones on Earth as of Spring, 2001. That's not a typo. I may have missed a few.

1Most of Arizona doesn't observe Daylight Saving Time either; thanks to some_guy for reminding me.

There are 25 time zones, all 15° in longtitude apart. All time zones are relative to Greenwich Mean Time, as measured in Greenwich, England. These are the standard, winter time, non-daylight savings, time zones.

GMT     Civilian                   Cities
+00:00  GMT (Greenwich Mean),    London, Dublin,
        UT(C), (Universal        Edinburgh, Lisbon,
            (Co-ordinated))        Reykjavik, Casablanca
        WET (Western European).
-01:00  WAT (West Africa)        Azores.
-02:00  AT (Azores)
-03:00                             Brazilia, Buenos Aires,
-04:00  AST (Atlantic Standard)  Caracas
-05:00  EST (Eastern Standard)   Bogoto, Lima, New York
-06:00  CST (Central Standard)   Mexico City, Saskatchewan
-07:00  MST (Mountain Standard)  
-08:00  PST (Pacific Standard)   Los Angeles
-09:00  YST (Yukon Standard)
-10:00  AHST (Alaska-Hawaii Standard)
        CAT (Central Alaska)
        HST (Hawaii Standard)
        EAST (East Australian Standard)
-11:00  NT (Nome)
-12:00  IDLW (International Date Line West)
+01:00  CET (Central European)   Paris, Berlin, Amsterdam,
        FWT (French Winter)      Brussels, Vienna, Madrid,
        MET (Middle European)    Rome, Bern, Stockholm, Oslo
        MEWT (Middle European Winter)
        SWT (Swedish Winter)
+02:00  EET (Eastern European,   Athens, Helsinki, Istanbul,
               Russia Zone 1)      Jerusalem, Harare, Zimbabwe
+03:00  BT (Baghdad, Russia      Kuwait, Nairobi, Riyadh,
              Zone 2)              Moscow
+04:00  ZP4 (Russia Zone 3)      Abu Dhabi, Muscat, Tblisi,
                                   Volgograd, Kabul
+05:00  ZP5 (Chesapeake Bay)
+06:00  ZP6 (Chesapeake Bay)
+07:00  WAST (Western Australian Standard)
+08:00  CCT (China Coast, Russia Zone 7)
+09:00  JST (Japan Standard, Russia Zone 8)
+10:00  GST (Guam Standard, Russia Zone 9)
+12:00  IDLE (International Date  Wellington, Fiji, 
                Line East)          Marshall Islands
        NZST (New Zealand Standard)
        NZT (New Zealand)
This nodeshell rescue was brought to you by Greenwich 2000 (which was utterly wrong about military time zones. see below)

Just as a matter of interest...

At the exact point where all 24 zones cross at the North and South poles, the official time zone is Greenwich Mean Time. This is also the time zone on the moon.

In its infinite wisdom, the United States Navy has assigned a letter of the alphabet to each time zone. Contrary to what one might expect, the zones don't start at "A" and wrap continuously around the earth. To add to the confusion, each time zone is referred to by its "phonetic" name, used to avoid confusion over the radio: This isn't the most intuitive system:
  • "Uniform" time refers to the US Pacific Coast.  Nothing uniform about that.
  • The Zulus aren't in Zulu Time. Bravo for them.
  • Quebec isn't in "Quebec Time" although its neighbor, Newfoundland, is.
  • Argentina isn't in Tango Time, but Salt Lake City is.
  • And why are all of those folks in Myanmar doing the Foxtrot?
  • If I were to make a date to meet some girl at 8:00, Romeo Time, she'd probably hang up.
  • And finally, for a tragic story about Zulu Time, follow the link.

To add to the confusion, the Navy published a map for years that incorrectly labeled the zones.

The FAA has picked up this system, so the world's entire civil aviation system has to understand it.

Gritchka posits a plausible theory about the origin of the system: "Z" stands for "Zero". The Navy may have picked up the British usage of "1200 h Z" for GMT and generalized it.


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