Time Zone Difference

Calculates the offset in hours and minutes between two named time zones for a given moment. Account is taken of daylight saving rules using the IANA tz database, so Sydney–London differs by 9, 10, or 11 hours depending on which side of DST transitions the date falls.

Fractional offsets are supported (Nepal at +5:45, India at +5:30).

The IANA Time Zone Database (also called tzdata, the Olson database, or the zoneinfo database) is the authoritative reference for time zone rules globally. It's updated several times per year as governments change daylight saving rules, redefine zone boundaries, or adjust UTC offsets. The database tracks current rules and historical transitions back to 1970 (the Unix epoch) for major zones and to 1900 or earlier for some, which means historical timestamp conversions remain accurate for most zones. Modern operating systems and browsers ship with the IANA database and consult it for any time zone calculation; updating the OS regularly keeps DST transitions correct.

A worked example: the offset between Asia/Tokyo and America/New_York. Tokyo does not observe DST and is permanently UTC+9. New York observes DST: EST (UTC−5) from early November to mid-March, EDT (UTC−4) from mid-March to early November. The Tokyo-to-New York difference is therefore 14 hours during EST winter and 13 hours during EDT summer. A meeting at 9 AM Tokyo time on January 15 happens at 19:00 the previous day in New York; the same 9 AM Tokyo on July 15 happens at 20:00 the previous day in New York. The single-hour shift catches out scheduling tools that use stale offsets.

Fractional offsets exist for several jurisdictions. India is UTC+5:30 (the half-hour offset adopted at independence to bridge the country's east-west span without splitting into two zones). Newfoundland is UTC−3:30. Nepal is UTC+5:45 (chosen to align solar noon with civil noon at Kathmandu's longitude). Chatham Islands (NZ) are UTC+12:45. Roughly 10% of the world's population lives in a non-integer-offset zone. Time zone calculations that assume integer offsets break for these regions.

Limitations and DST ambiguities are real edge cases. On 'fall back' day, when clocks rewind from 2:00 AM to 1:00 AM, local times between 1:00 and 2:00 occur twice. A meeting scheduled at 1:30 AM is ambiguous; software that doesn't track UTC anchors can misinterpret. On 'spring forward' day, clocks jump from 2:00 to 3:00, and the 2:00-3:00 hour does not exist; scheduling for 2:30 AM in that hour produces an undefined or skipped time. UTC anchoring (storing 'meeting at 14:00 UTC' rather than 'meeting at 9 AM Eastern') eliminates this entirely. Recurring meetings that appear in calendars as 'every Monday at 10 AM Eastern' actually shift by one hour relative to UTC twice yearly when DST transitions; participants in non-DST zones see the meeting time wander.