What is the difference between UTC and GMT?

UTC (Coordinated Universal Time) is the primary time standard by which the world regulates clocks, defined by atomic clocks and maintained by the BIPM. GMT (Greenwich Mean Time) is a time zone, not a standard, based on mean solar time at the Prime Meridian. In practice they share the same offset (UTC+0) during winter, but GMT is an informal convention while UTC is the scientific standard used in computing, aviation, and telecommunications. Most programming libraries and databases store times as UTC internally.

Why do some timezone conversions show unexpected dates?

When converting across the International Date Line (roughly 180° longitude), the calendar date changes. For example, converting noon Monday in Los Angeles (UTC-8) to Tokyo (UTC+9) gives 5:00 AM Tuesday — a 17-hour gap that crosses midnight. The IANA timezone database encodes these date-crossing rules precisely. Always use full date-time values, not just times, when communicating across timezone boundaries.

How does Daylight Saving Time affect conversions?

Daylight Saving Time (DST) shifts clocks forward by one hour in spring and back in autumn, but the exact dates differ by country and even region. The United States moves clocks on the second Sunday of March, while the EU moves on the last Sunday of March. Some regions (Arizona, most of Asia, Africa) observe no DST at all. This means the offset between two timezones can change twice a year, making IANA tz database lookups essential for accurate conversions rather than hardcoded offsets.

What does the IANA timezone database contain?

The IANA Time Zone Database (also called the Olson database or tzdata) is the authoritative repository of historical and future timezone rules for every inhabited region on Earth. It defines over 500 timezone identifiers in the format Region/City (e.g., Europe/Paris) and records every DST transition, UTC offset change, and timezone boundary shift ever made. Operating systems, browsers, and programming languages ship tzdata internally and update it when governments change their timezone rules. Using IANA identifiers rather than abbreviations like EST or IST avoids ambiguity, since many abbreviations refer to multiple different timezones.

Can a timezone converter handle half-hour and 45-minute offsets?

Yes. Several countries use non-standard offsets: India (IST) is UTC+5:30, Nepal is UTC+5:45, Iran is UTC+3:30, and Australia's Lord Howe Island is UTC+10:30 in winter and UTC+11 in summer. These offsets are fully encoded in the IANA database. A compliant timezone converter handles them transparently — the user just selects Asia/Kolkata or Asia/Kathmandu and the correct fractional offset is applied automatically.

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Part of the FYIPedia open-source developer tools ecosystem. Hub: ToolFYI

Site	Domain	Focus
UnitFYI	unitfyi.com	Unit conversion for 220 units across 20 categories
TimeFYI	timefyi.com	World clock, timezone converter, sunrise & sunset times
HolidayFYI	holidayfyi.com	Public holidays & observances for 200+ countries
NameFYI	namefyi.com	Name origins, romanization, and cultural naming traditions