Daylight saving time explained
Why clocks change, why countries disagree on dates and how DST affects international time differences.
DST changes the UTC offset
A city can use one UTC offset in standard time and another during daylight saving. That means the time difference between two cities can change even if neither city moves geographically.
Transition dates do not always match
North America and Europe often switch on different weekends. For a few weeks each year, a familiar transatlantic time difference can therefore be one hour different.
Some countries do not use DST
Many countries keep a constant offset all year. Rules can also change politically, which is another reason to use maintained IANA time-zone identifiers.
Scheduling rule
Never schedule a future international meeting by adding today’s offset manually. Convert the exact future date with the city’s named zone.