Standard time zones can be defined by geometrically subdividing the Earth's spheroid into २४ lunes (wedge-shaped sections), bordered by meridians each १५° of longitude apart. The local time in neighbouring zones would differ by one hour. However, political र geographical practicalities can result in irregularly-shaped zones that follow political boundaries or that change their time seasonally (as with daylight saving time), as well as being subject to occasional redefinition as political conditions change.
There are variations of the definitions of time zone which generally fall into two meanings: a समय क्षेत्र can represent a region where the local time is some fixed offset from a global reference (usually UTC), or a समय क्षेत्र can represent a region throughout which the local time is always consistent even though the offset may fluctuate seasonally.
Before the adoption of time zones, people used local solar time (originally apparent solar time, as with a sundial; and, later, mean solar time). Mean solar time is the average over a year of apparent solar time. Its difference from apparent solar time is the equation of time.
This became increasingly awkward as railways र telecommunications improved, because clocks differed between places by an amount corresponding to the difference in their geographical longitude, which was usually not a convenient number. This problem could be solved by synchronizing the clocks in all localities, but then in many places the local time would differ markedly from the solar time to which people are accustomed. Time zones are thus a compromise, relaxing the complex geographic dependence while still allowing local time to approximate the mean solar time. There has been a general trend to push the boundaries of time zones further west of their designated meridians in order to create a permanent daylight saving time effect. The increase in worldwide communication has further increased the need for interacting parties to communicate mutually comprehensible time references to one another.
Standard time zones[सम्पादन करी]
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 clocks was annually changed or steered to closely match GMT. But on January 1, 1972 it became fixed, using predefined leap seconds 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 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, United States: UTC-8; ०४:००
- Saint John, New Brunswick, Canada: UTC-4; ०८:००
- Toronto, Ontario, Canada: UTC-5; ०७:००
- Stockholm, Sweden: UTC+1; १३:००
- Cape Town, South Africa: UTC+2; १४:००
- Mumbai, India: UTC+5:30; १७:३०
- Seoul, Korea: UTC+9; २१:००
- Melbourne, 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:
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.