Which time zone is used in New Zealand's main islands during standard time?

New Zealand Standard Time is used in New Zealand’s main islands during standard time.

NZST is 12 hours ahead of Coordinated Universal Time, or UTC+12:00. It applies to the North Island and South Island, along with smaller islands that follow mainland New Zealand’s time rules.

New Zealand officially adopted a national standard time on 2 November 1868, based initially on the 172°30′ east meridian. The modern UTC+12:00 definition comes from the Time Act 1974, which defines New Zealand Standard Time as 12 hours ahead of UTC.

A frequent mix-up is with Chatham Standard Time, used in the Chatham Islands, which is 45 minutes ahead at UTC+12:45. During the southern summer, the main islands advance one hour to New Zealand Daylight Time, UTC+13:00, so NZST specifically refers to standard-time months.