What time zone does London use for its New Year's Eve countdown in winter?

London uses Greenwich Mean Time for its New Year's Eve countdown in winter.

The United Kingdom switches from British Summer Time to Greenwich Mean Time when daylight saving time ends in autumn. Because December 31 falls during this winter period, London is on GMT at midnight. GMT is conventionally UTC+0, so London's midnight occurs at the same instant as midnight in locations observing UTC, although countries elsewhere may enter the new year earlier or later.

The time standard takes its name from Greenwich, in southeast London, where the Royal Observatory established the historic prime meridian reference. Modern international timekeeping formally uses Coordinated Universal Time, or UTC, but GMT remains the familiar civil-time label used in Britain during winter.

A common mix-up is applying British Summer Time to New Year's Eve. BST is UTC+1 and is used during the warmer part of the year, not in December. London clocks therefore show GMT during the fireworks and countdown on December 31.