Timezone Converter for APIs

Offsets are not time zones

UTC+1 is an offset. Europe/Paris is a time zone: a set of rules describing which offset applies at which moment, including daylight saving transitions and every historical change. Storing "UTC+1" loses the rules, so a meeting saved that way silently moves by an hour when the clocks change. Always store the IANA zone identifier alongside the time, not the offset it happened to have.

Store UTC, convert at display

The rule that avoids most bugs: persist instants in UTC, convert to the user's zone only when rendering. A timestamp stored in local time becomes ambiguous the moment the zone's rules change, and governments change them with weeks of notice. UTC has no daylight saving and no political revisions.

Future events are the exception

A recurring 9 a.m. Monday meeting is not a fixed instant in UTC, because the UTC time it corresponds to shifts when either participant's zone enters or leaves daylight saving. Future scheduled events must store the local wall-clock time plus the zone identifier, and be resolved to an instant at the point of use. Storing them as UTC is how meetings drift by an hour twice a year.

Two impossible hours a year

When clocks spring forward, an hour does not exist: 02:30 on that date is not a valid local time, and constructing it throws or silently shifts depending on the library. When clocks fall back, an hour occurs twice, so 01:30 is ambiguous and needs a rule to pick the first or second occurrence. Any code handling user-entered local times must decide what to do in both cases, because doing nothing means whatever the library defaults to.

The rules change, so the database must be updated

The IANA time zone database is revised several times a year as countries alter their daylight saving policy. Applications carrying a stale copy compute wrong times for affected regions until updated. This is a maintenance obligation, not a one-time setup, and it bites hardest in containers and mobile apps that pin dependencies.

Abbreviations are ambiguous

CST means Central Standard Time in North America, China Standard Time and Cuba Standard Time. IST covers India, Ireland and Israel. There are far more abbreviations in circulation than distinct meanings, and no authority governing them. Use IANA identifiers such as America/Chicago or Asia/Kolkata in anything machine-read; abbreviations belong in display text only.