Daylight saving time, explained
Why your usual time difference with another city sometimes shifts by an hour, twice a year.
Daylight saving is the single biggest source of time-zone mistakes, because regions change on different dates and some do not change at all.
When clocks change
In North America, clocks spring forward in March and fall back in November. In Europe the shifts fall on the last Sundays of March and October. Because these dates do not line up, there are short windows each year when the offset between, say, New York and London is an hour different from the rest of the year.
Who opts out
Plenty of places ignore daylight saving entirely, including most of Arizona, Hawaii, Japan, China and India. Near the equator, day length hardly changes, so there is little reason to shift the clocks.
The southern hemisphere runs opposite
When the north springs forward into summer, the south is heading into autumn and shifting the other way. For someone coordinating between, for example, London and Sydney, the time difference can swing by two hours across the year.
Frequently asked questions
When does daylight saving start and end?
In North America, clocks spring forward in March and fall back in November. Europe shifts on the last Sundays of March and October. Exact dates differ between regions.
Who does not observe daylight saving?
Many places skip it, including most of Arizona, Hawaii, Japan, China, India, and most equatorial countries where day length barely changes.
Why does daylight saving complicate scheduling?
Because regions change on different dates, or not at all, the usual offset between two cities can shift for a week or two each spring and autumn, catching schedulers out.
Does the southern hemisphere change opposite to the north?
Yes. When the northern hemisphere springs forward, the southern hemisphere is moving into autumn and shifting the other way, widening or narrowing offsets.
How do your tools handle these changes?
Automatically, using each region's official time-zone rules for the date you select, so conversions stay correct across transitions.