UTC, not GMT

World clocks run on UTC (Coordinated Universal Time), not GMT (Greenwich Mean Time), even though the two usually read the same. GMT was originally set by the sun’s position over Greenwich, which drifts slightly with Earth’s rotation; UTC is kept by atomic clocks instead, corrected with occasional leap seconds. The UK still calls its winter time “GMT,” but for a precise reference, UTC is the name to use.

Daylight saving doesn’t start on the same day everywhere

The US moves clocks forward on the second Sunday of March, back on the first Sunday of November. The EU shifts on the last Sunday of March and October. Because the dates don’t match, New York and London — normally five hours apart — narrow to four for a week or two each spring, until Europe catches up.

Why picking a date matters for a meeting

This tool works out the offset for the date you enter, not for today. A meeting on the far side of a clock change won’t share today’s offset, so timing it from today’s gap can leave you an hour off. Pick the date first, then check its offset.

Not every zone sits on the hour

India runs on UTC+05:30 and Nepal on UTC+05:45, off the whole hour by a half or three-quarter hour. Each country balances true solar position against having one time across its territory, and the result sometimes lands off the hour.

Why zone names are cities, not offsets

A zone like Asia/Seoul is named after a place, not an offset like UTC+9, because a country can change its daylight saving rule without renaming the zone. The city name keeps pointing at the same place after the rule underneath it changes.

Places English speakers check often

“GMT” alone is a poor shorthand once you’re scheduling widely — London itself leaves it every summer. The US East and West Coasts already span three hours, and London or Singapore adds most of a workday on top. Add each city to this tool once instead of recomputing the gap by hand every time daylight saving flips.