New Zealand Mean Time
Time in New Zealand, by law, is divided into two standard time zones. The main islands use New Zealand Standard Time (NZST), 12 hours in advance of Coordinated Universal Time (UTC) / military M (Mike), while the outlying Chatham Islands use Chatham Standard Time (CHAST), 12 hours 45 minutes in advance of UTC / military M^ (Mike-Three).
During summer months—from the last Sunday in September until the first Sunday in April—daylight saving time is observed and clocks are advanced one hour. New Zealand Daylight Time (NZDT) is 13 hours ahead of UTC, and Chatham Daylight Time (CHADT) 13 hours 45 minutes ahead.
New Zealand dependencies, the Cook Islands, Niue and Tokelau, use several different times zones at their own discretion.
On 2 November 1868, New Zealand officially adopted a standard time to be observed nationally, and was perhaps the first country to do so. It was based on longitude 172° 30′ East of Greenwich, 11 1⁄2 hours ahead of Greenwich Mean Time (GMT). This standard was known as New Zealand Mean Time (NZMT).
In 1941, during the Second World War, clocks were advanced half an hour, making New Zealand 12 hours ahead of GMT. This change was made permanent from 1946 by the Standard Time Act 1945, at which the time at the 180°E meridian was made the basis for New Zealand Time. NZST remained half an hour ahead of NZMT, and the Chatham Islands 45 minutes ahead of NZST.
In the late 1940s the atomic clock was developed and several laboratories began atomic time scales. A new time scale known as Coordinated Universal Time (UTC) was adopted internationally in 1972. This was based on the readings of atomic clocks, updated periodically in accordance with time variations in the Earth's rotation by the addition or deletion of seconds (called leap seconds). The Time Act 1974 defines New Zealand Standard Time as 12 hours in advance of UTC.
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