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[[Image:Standard time zones of the world.png|thumb|400px|विश्वको कानक समय २०११ को अनुसार]]
== Standard time zones ==
Earlier, time zones based their time on Greenwich Mean Time (GMT, also called [[UT1]]), the mean solar time at longitude ०° (the [[Prime Meridian]]). But as a mean solar time, GMT is defined by the rotation of the Earth, which is not constant in rate. So, the rate of [[atomic clock]]s was annually changed or steered to closely match GMT. But on [[January 1]], [[1972]] it became fixed, using predefined [[leap second]]s instead of rate changes. This new time system is [[Coordinated Universal Time]] (UTC). Leap seconds are inserted to keep UTC within ०.९ seconds of UT१. In this way, local times continue to correspond approximately to mean solar time, while the effects of variations in Earth's rotation rate are confined to simple step changes that can be easily subtracted if a uniform [[time scale]] ([[International Atomic Time]] or TAI) is desired. With the implementation of UTC, nations began to use it in the definition of their time zones instead of GMT. As of २००५, most but not all nations have altered the definition of local time in this way (though many media outlets fail to make a distinction between GMT र UTC). Further change to the basis of time zones may occur if proposals to abandon leap seconds succeed.
Due to [[daylight saving time]], UTC is local time at the [[Royal Observatory, Greenwich]] only between ०१:०० UTC on the last Sunday in October र ०१:०० UTC on the last Sunday in March. For the rest of the year, local time there is [[UTC+1]], known in the [[संयुक्त अधिराज्य|United Kingdom]] as [[British Summer Time]] (BST). Similar circumstances apply in many places.
The definition for time zones can be written in short form as UTC±''n'' (or GMT±''n''), where ''n'' is the offset in hours. These examples give the local time at various locations at १२:०० UTC when daylight saving time (or summer time, etc.) is not in effect:
* [[San Francisco, California|San Francisco]], [[California]], [[संयुक्त राज्य अमेरिका|United States]]: [[UTC-8]]; ०४:००
* [[Saint John, New Brunswick]], [[Canada]]: [[UTC-4]]; ०८:००
* [[Toronto, Ontario|Toronto]], [[Ontario]], [[Canada]]: [[UTC-5]]; ०७:००
* [[Stockholm]], [[Sweden]]: [[UTC+1]]; १३:००
* [[Cape Town]], [[South Africa]]: [[UTC+2]]; १४:००
* [[Mumbai]], [[भारत|India]]: [[UTC+5:30]]; १७:३०
* [[Seoul]], [[कोरिया|Korea]]: [[UTC+9]]; २१:००
* [[Melbourne]], [[अष्ट्रेलिया|Australia]]: [[UTC+10]]; २२:००
Where the adjustment for time zones results in a time at the other side of midnight from UTC, then the date at the location is one day later or earlier. Some examples when UTC is २३:०० on Monday when daylight saving time is not in effect:
* [[Cairo]], [[मिश्र|Egypt]]: UTC+२; ०१:०० on Tuesday
* [[Wellington]], [[New Zealand]]: [[UTC+12]]; ११:०० on Tuesday
Some examples when UTC is ०२:०० on Tuesday when daylight saving time is not in effect:
* [[New York City]], [[New York]], United States: [[UTC-5]]; २१:०० on Monday
* [[Honolulu]], [[Hawaii]], United States: [[UTC-10]]; १६:०० on Monday
The time-zone adjustment for a specific location may vary because of Daylight Saving Time. For example New Zealand, which is usually [[UTC+12]], observes a one-hour daylight saving time adjustment during the [[southern hemisphere]] summer, resulting in a local time of [[UTC+13]].
See also [[Sidereal time]].
[[श्रेणी:भूगोल]]
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