Who observes daylight saving time?
Daylight saving changes the civil clock for part of the year. Many places move their clocks forward for a summer period and back afterwards. Many others keep the same offset all year. Even places that observe it do not necessarily change on the same date.
The rules belong to jurisdictions, and governments can revise them. A reliable conversion needs a place and a date, supported by an up-to-date time-zone database.
The skipped hour in spring
When a clock moves forward, some local times do not occur. Suppose a clock jumps directly from 01:59:59 to 03:00:00. On that date, 02:30 is not a real local time in that zone.
Software sometimes silently moves that appointment to a later hour. Our converter instead tells you the time is skipped, so you can choose a time that actually occurs.
The repeated hour in autumn
When a clock moves back, part of the local day repeats. A reading such as 01:30 may describe two separate instants, with different UTC offsets.
Our converter lets you choose the first or second occurrence. The first is the earlier instant. A calendar export then records a specific UTC instant so the ambiguity does not travel with your invitation.
Why the time difference between cities changes
New York and London change their clocks on different schedules. For parts of the year, their usual difference changes temporarily. Sydney observes summer time in the southern hemisphere, so its seasonal pattern is opposite to much of Europe and North America.
Even a city that never changes clocks has changing differences from cities that do. Tokyo’s UTC offset can stay fixed while its difference from London changes.
Tips for planning around clock changes
- Use a named city instead of an ambiguous abbreviation.
- Check the actual date of the event, not just the time difference today.
- Keep browsers, operating systems, and calendar software updated.
Further reading
Source: the IANA time-zone database, which tracks changes to offsets and daylight-saving rules.