How Daylight Saving Time Works
Understand clock changes, skipped and repeated local times, changing UTC offsets and the practical effect of daylight saving on travel and meetings.
Daylight saving time (DST) is a seasonal change to civil clocks used by some jurisdictions. It does not change Coordinated Universal Time (UTC); it changes the local offset from UTC.
What “spring forward” actually does
When clocks move forward, one block of local clock labels is skipped. In the common US transition, 1:59:59 AM is followed by 3:00:00 AM, so a local time such as 2:30 AM does not occur that day. When clocks move back, an hour is repeated, so a local label such as 1:30 AM occurs twice with different UTC offsets.
The 2026 United States dates
For most US locations that observe DST, the NIST 2026 schedule runs from 8 March at 2:00 AM local time to 1 November at 2:00 AM local time. Hawaii and most of Arizona are among the exceptions. Other countries use different dates, and many do not change clocks.
Why international time differences shift
If New York changes offset while Singapore does not, their difference changes by one hour. The change may happen again when New York returns to standard time. Europe follows a different transition calendar, creating short periods when a familiar US–Europe difference is also one hour off.
Use the date, city and wall-clock time together
A fixed statement such as “London is five hours ahead of New York” is not safe for every date. Select the intended date and cities in the Terbit Time Zone Converter. For several participants, use the Meeting Planner and review the result again before a future event.
What happens to alarms and appointments near a transition?
- A time inside the skipped spring hour may be rejected, shifted or interpreted differently by calendar software.
- A time inside the repeated autumn hour is ambiguous unless the time zone and offset are retained.
- A recurring meeting may remain fixed for the organizer but move by one hour for participants elsewhere.
- A duration such as “in two hours” is different from a wall-clock instruction such as “at 2:30 AM.”
Keep devices and rules current
The IANA Time Zone Database is updated when political bodies change time-zone boundaries, UTC offsets or daylight-saving rules. Those updates must then reach operating systems and browsers. An old or unsupported device can therefore disagree with newer software after a rule change.
Frequently asked questions
Does every country use daylight saving time?
No. Governments decide their civil-time rules, and many places keep one UTC offset all year.
Does UTC move when DST starts?
No. UTC remains the international reference; the local zone's offset changes.
Can future DST rules change?
Yes. Treat distant-future conversions as provisional and recheck them after device and time-zone database updates.