World Clock

A Brief History of World Time

As the United Kingdom grew into an advanced maritime nation, British mariners kept at least one timepiece on GMT in order to calculate their longitude from the Greenwich meridian, which was by convention considered to have longitude zero degrees. This did not affect shipboard time itself, which was still solar time. This, combined with mariners from other nations drawing from Nevil Maskelyne's method of lunar distances based on observations at Greenwich, eventually led to GMT being used world-wide as a reference time independent of location. Most time zones were based upon this reference as a number of hours and half-hours "ahead of GMT" or "behind GMT".

Greenwich Mean Time was adopted across the island of Great Britain by the Railway Clearing House in 1847, and by almost all railway companies by the following year. It was gradually adopted for other purposes, but a legal case in 1858 held "local mean time" to be the official time. This changed in 1880, when GMT was legally adopted throughout the island of Great Britain. GMT was adopted on the Isle of Man in 1883, Jersey in 1898 and Guernsey in 1913. Ireland adopted Greenwich Mean Time in 1916, supplanting Dublin Mean Time. Hourly time signals from Greenwich Observatory were first broadcast on 5 February 1924.

The daily rotation of the Earth is somewhat irregular and is slowing down slightly. Atomic clocks constitute a much more stable timebase. On 1 January 1972, GMT was replaced as the international time reference by Coordinated Universal Time, maintained by an ensemble of atomic clocks around the world. UT1, introduced in 1928, represents earth rotation time. Leap seconds are added to or subtracted from UTC to keep it within 0.9 seconds of UT1.

The international prime meridian is no longer precisely the Greenwich meridian, but remains close to it (5.31"E).

  Local Time     Greenwich Mean Time (GMT)  

West of Greenwich           East of Greenwich

Azores (GMT-1)       (GMT+1) Rome
Mid-Atlantic (GMT-2)       (GMT+2) Israel
Brazilia (GMT-3)       (GMT+3) Moscow
Atlantic Time (GMT-4)       (GMT+4) Baku
Eastern Time (GMT-5)       (GMT+5) New Delhi
Central Time (GMT-6)       (GMT+6) Dhakar
Mountain Time (GMT-7)       (GMT+7) Bangkok
Pacific Time (GMT-8)       (GMT+8) Hong Kong
Alaska (GMT-9)       (GMT+9) Tokyo
Hawaii (GMT-10)       (GMT+10) Sydney
Samoa (GMT-11)       (GMT+11) Magadan
Eniwetok (GMT-12)       (GMT+12) Wellington

Definition of time zones

A time zone item specifies an international time zone, indicated by a small set of letters.

Military time zone designations use a single letter (except j).  Zulu Time (z) represents Greenwich Mean Time (GMT).

Here are the most common standard-time time zones, indexed by the zone hour value.

Time Zone Abbreviations
West of Greenwich
-0:00 GMT = Greenwich Mean Time
UT or UTC for Universal (Coordinated) Time
WET for Western European Time
-1:00 WAT = West Africa Time a
-2:00 AT = Azores Time b
-3:00   c
-4:00 AST = Atlantic Standard Time (Canada) d
-5:00 EST = Eastern Standard Time (USA & Canada) e
-6:00 CST = Central Standard Time (USA & Canada) f
-7:00 MST = Mountain Standard Time (USA & Canada) g
-8:00 PST = Pacific Standard Time (USA & Canada) h
-9:00 AKST Alaska Standard Time (USA)
YST = Yukon Standard Time (Canada)
-10:00 HST = Hawaii Standard Time
HAST Hawaii-Aleutian Standard Time (USA)
AHST = Alaska-Hawaii Standard Time (obs)
CAT = Central Alaska Time
-11:00 NT = Nome Time l
-12:00 IDLW = International Date Line West m

Time Zone Abbreviations
East of Greenwich


+1:00 CET = Central European Time
FWT = French Winter Time (France)
MET = Middle European Time
MEWT = Middle European Winter Time
SWT = Swedish Winter Time (Sweden)
+2:00 EET = Eastern European Time, USSR Zone 1 o
+3:00 BT = Baghdad Time, USSR Zone 2 p
+4:00 ZP4 = USSR Zone 3 q
+5:00 ZP5 = USSR Zone 4. r
+6:00 ZP6 = USSR Zone 5. s
+7:00 CXT = Christmas Island Time (Australia) t
+8:00 CCT = China Coast Time, USSR Zone 7
AWST = Australian Western Standard Time
WST = Western Standard Time (Australia)
+9:00 JST = Japan Standard Time, USSR Zone 8 v
+10:00 EAST = East Australian Standard Time
EST = Eastern Standard Time (Australia)
GST = Guam Standard Time, USSR Zone 9
+11:00   x
+12:00 IDLE = International Date Line East
NZST = New Zealand Standard Time
NZT = New Zealand Time
Daylight Saving Time (DST) time zones, indexed by the zone hour value.

Time Zone Abbreviations
East of Greenwich


-3:00 ADT = Atlantic Daylight Time (Canada) c
-4:00 EDT = Eastern Daylight Time (USA & Canada) d
-5:00 CDT = Central Daylight Time (USA & Canada) e
-6:00 MDT = Mountain Daylight Time (USA & Canada) f
-7:00 PDT = Pacific Daylight Time (USA & Canada) g
-8:00 AKDT = Alaska Daylight Time (USA)
YDT = Yukon Daylight Time (Canada)
HADT = Hawaii-Aleutian Daylight Time (USA)
-9:00 HDT = Hawaii Daylight Time (USA) i

Time Zone Abbreviations
West of Greenwich
+1:00 BST = British Summer Time (UK)
IST = Irish Summer Time (Ireland)
WEDT = Western European Daylight Time
WEST Western European Summer Time
+2:00 CEDT = Central European Daylight Time
CEST = Central European Summer Time
MEST = Middle European Summer Time
MESZ = Middle European Summer Time
SST = Swedish Summer Time (Sweden)
FST = French Summer Time (France)
+3:00 EEDT = Eastern European Daylight Time
EEST Eastern European Summer Time
+8:00 WADT = West Australian Daylight Time  
+11:00 EADT = Eastern Australian Daylight Time
EST = Eastern Summer (Daylight) Time (Australia)
+13:00 NZDT = New Zealand Daylight Time n/a

French Time Zone Abbreviations

HAA = Heure Avancée de l'Atlantique GMT-3
HAC = Heure Avancée du Centre GMT-5
HAE = Heure Avancée de l'Est GMT-4
HAP = Heure Avancée du Pacifique GMT-7
HAR = Heure Avancée des Rocheuses GMT-6
HAT = Heure Avancée de Terre-Neuve GMT-2:30
HAY = Heure Avancée du Yukon GMT-8
HNA = Heure Normale de l'Atlantique GMT-4
HNC = Heure Normale du Centre GMT-6
HNE = Heure Normale de l'Est GMT-5
HNP = Heure Normale du Pacifique GMT-8
HNR = Heure Normale des Rocheuses GMT-7
HNT = Heure Normale de Terre-Neuve GMT-3:30
HNY = Heure Normale du Yukon GMT-9

German Time Zone Abbreviations

MESZ = Mitteleuropäische Sommerzeit GMT+2
MEZ = Mitteleuropäische Zeit GMT+1

Non-standard Time Zones

ACDT Australian Central Daylight Time Australia GMT+10:30
ACST Australian Central Standard Time Australia GMT+9:30

CST Central Summer (Daylight) Time Australia GMT+10:30
CST Central Standard Time Australia GMT+9:30

IST = India Standard Time GMT+5:30

NDT Newfoundland Daylight Time (Canada) GMT-2:30
NFT Norfolk (Island) Time (Australia) GMT+11:30
NST Newfoundland Standard Time (Canada) GMT-3:30

Military Time Zones

The hourly zones are used by the military are referred to by a letter of the alphabet (or their phonetic equivalent).

The letter J is not used.

The other 25 letters cover the 12 time zones to the east of Greenwich and the 12 time zones to the west of Greenwich.  The 25th zone is Z (Zulu).  Zulu is the same as GMT or UTC.

A - M are for time zones to the East of Greenwich
N - Y are for time zones to the West of Greenwich.

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