How time zones and daylight saving work

Time zones are defined as offsets from Coordinated Universal Time (UTC). New York is UTC−5 in winter, London is UTC+0 and India is UTC+5:30. Not every offset is a whole number of hours.

A name like America/New_York identifies a region and its rules, which is more useful than a fixed offset because the offset can change during the year.

Many regions move their clocks forward in spring and back in autumn. While they do, their offset from UTC changes by an hour.

Regions switch on different dates. The US changes on the second Sunday of March, while Europe changes on the last Sunday, so for a few weeks the gap between New York and London is 4 hours instead of the usual 5.

Convert on the actual date of the meeting, not on today's date. A call at 09:00 in New York in January is 14:00 in London, but the same call in late March may be 13:00.

When scheduling across regions, share the time with its zone name and check the date, particularly near clock changes.

Many countries, including India, Pakistan, Japan and China, do not change their clocks. Their offset stays the same all year, but the gap to regions that do change will move.

That is why a fixed rule such as add 5 hours is unreliable. A converter that uses the real rules for each date is always safer.