Daylight saving time & time zones, explained

Daylight saving time (DST) moves clocks forward (usually by an hour) for part of the year. A place's UTC offset therefore depends on the date: New York is UTC−05:00 in winter and UTC−04:00 in summer. That's why a time zone is a set of rules (identified by an IANA name like America/New_York), not a fixed offset.

Nonexistent times. When clocks jump forward, the skipped wall-clock times never happen. In New York on spring-forward day, 2:30 AM doesn't exist. Ambiguous times. When clocks go back, an hour repeats: 1:30 AM happens twice. Software that silently picks one is a common source of bugs — Time always asks.

Hemispheres differ. Australia and New Zealand use DST during their summer — the northern winter. For a few weeks each spring and autumn, gaps between cities shift by an hour because countries change on different dates.

UTC vs GMT. UTC is the world's time standard. GMT is a time zone name (the UK in winter) that happens to share UTC's offset; in summer the UK is on BST, UTC+01:00. Time uses UTC for references.

Abbreviations are dangerous. IST is India, Ireland or Israel; CST is US Central, China or Cuba; BST is British Summer Time or Bangladesh. Time shows abbreviations only as hints next to the city, offset and IANA zone.

In 2026, New York changes on 8 Mar 2026 and 1 Nov 2026; Sydney on 5 Apr 2026 and 4 Oct 2026 — opposite seasons, opposite directions.

On 1 November 2026, 1:30 AM happens twice in New York: first at 1:30 AM EDT (UTC-04:00), then an hour later at 1:30 AM EST (UTC-05:00).

• Zones with no DST (India, Japan, most of Africa, Arizona) list no changes.
• 30- and 45-minute offsets (India, Nepal, Newfoundland).
• Countries change DST on different dates, so city gaps shift temporarily.
• Legacy zone names (Asia/Calcutta) are recognised and shown with current names.