Standard Time Act 1945

New Zealand has two 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 (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.

The Ross Dependency, McMurdo Station and the Amundsen–Scott South Pole Station in Antarctica maintain NZST/NZDT.

There are dependencies of New Zealand in the Pacific Ocean, in three different time zones, two of them on the other side of the International Date Line:

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.

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