World Clock

Why time zones resist simple arithmetic

A time zone is not an offset. It is a set of rules describing which offset applies on which dates, and those rules change: governments move daylight saving boundaries, adopt or abandon the practice, and occasionally shift zone entirely. The IANA time zone database exists to track this, is updated several times a year, and ships with every operating system and browser. Software that stores "UTC+1" against a city has captured a single moment of a moving target; software that asks the platform what the offset is at a specific instant gets an answer that accounts for whatever rules were in force. This tool does the latter, which is why it handles daylight saving, half-hour offsets and historical changes without having any knowledge of them itself.

The transitions are where the bugs live

Twice a year, most daylight-saving zones produce a local time that either does not exist or happens twice. When clocks spring forward, the hour between 01:00 and 02:00 is skipped entirely — a meeting scheduled at 01:30 that day has no corresponding instant. When they fall back, that hour repeats, so 01:30 is ambiguous and refers to two different moments an hour apart. Any calculation that converts a local time into an instant has to decide what to do about both cases. Reading in the other direction, as this tool does — taking a known instant and asking what each zone displays — avoids the problem entirely, because an instant is unambiguous by definition and every zone has exactly one answer for it.

Coordinating across zones

The practical difficulty in scheduling across regions is rarely the arithmetic; it is that the useful overlap between two working days can be very small, and that it moves. London and Sydney have almost none, and what little exists shifts by two hours across the year as the two hemispheres change clocks on opposite schedules. Seeing the whole board at once, with the local date alongside the local time, makes the shape of that constraint visible: which cities are awake now, which are already into tomorrow, and where the hours that work for everyone actually fall.