Time zones
What is daylight saving time?
Why clocks move, why the date differs by country and why a city’s offset cannot be assumed year-round.
A seasonal civil-time policy
Daylight saving time advances local clocks, usually by one hour, for part of the year. It changes the civil label attached to an instant; it does not create extra daylight or alter Earth’s rotation.
Transitions create unusual local times
During a spring-forward transition, a block of local clock readings does not occur. During a fall-back transition, a block repeats. That is why a date, city and time-zone rule are all required to identify some instants unambiguously.
Rules differ and can change
Countries choose their own start dates, end dates and exemptions, while many never use daylight saving time. Reliable converters use an updated time-zone database rather than assuming one universal schedule.