What is the difference between UTC and GMT?

UTC (Coordinated Universal Time) and GMT (Greenwich Mean Time) are nearly identical for everyday use: both sit at the zero meridian with no UTC offset. The technical difference is that GMT is a timezone based on mean solar time at the Royal Observatory in Greenwich, while UTC is an atomic time standard. Most programming contexts use UTC.

How does daylight saving time affect Unix timestamps?

Unix timestamps are not affected by daylight saving time because they are always measured in UTC seconds. DST only affects the display of a timestamp when it is converted to a local timezone. This converter applies the correct DST offset for the selected timezone and date automatically.

What is an IANA timezone identifier?

An IANA timezone identifier (also called an Olson timezone name) is a string like 'America/New_York' or 'Asia/Tokyo' that uniquely identifies a timezone region and its full history of UTC offsets and DST rules. They are maintained in the IANA Time Zone Database and supported by all major operating systems and programming languages.

Should I store UTC or local time in my database?

Always store UTC (or Unix timestamps) in your database. Store the user's timezone preference separately and convert to local time only at display time. This avoids ambiguity during DST transitions and makes cross-timezone comparisons reliable.

Timezone

Timezone Converter

Enter a date and time in any timezone and see it converted to multiple timezones at once. Covers all IANA regions with automatic daylight saving time handling.

Unix Timestamp — page opened

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seconds

Source time

Enter the date and time you want to convert.

Target timezones

The same moment shown in each timezone.

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IANA timezone database

This tool uses the IANA timezone database (also called the tz database or Olson database), the most comprehensive and authoritative source of timezone rules. It covers the full history of UTC offsets and daylight saving time transitions for every timezone region in the world. Your browser's built-in Intl API provides access to this data without any additional libraries or network requests.

Daylight saving time (DST)

Daylight saving time shifts clocks forward by one hour in spring and back in autumn for regions that observe it. This converter automatically applies the correct DST offset for the selected timezone and date. For example, America/New_York is UTC-5 during Eastern Standard Time (EST) and UTC-4 during Eastern Daylight Time (EDT).

United States DST: second Sunday in March → first Sunday in November
European DST: last Sunday in March → last Sunday in October
Many countries (including China, Japan, India) do not observe DST
Unix timestamps are not affected by DST — they are always counted in UTC seconds

UTC offsets explained

A UTC offset describes how many hours and minutes a timezone is ahead of or behind UTC. UTC+00:00 is UTC itself. UTC+08:00 is 8 hours ahead (e.g. China Standard Time, Singapore, Perth). UTC-05:00 is 5 hours behind (Eastern Standard Time). Offsets range from UTC-12:00 to UTC+14:00.

UTC+00:00 — UTC, GMT, West Africa Time
UTC+01:00 — Central European Time (CET), West Africa Standard Time
UTC+05:30 — India Standard Time (IST, no DST)
UTC+08:00 — China Standard Time, Singapore, Philippine Time
UTC+09:00 — Japan Standard Time, Korea Standard Time (no DST)
UTC-05:00 — Eastern Standard Time (UTC-4 during EDT)

The IANA tz database, Etc/GMT, and political timezone changes

Timezones are political, not geographic — and they change. The IANA timezone database (also called Olson or zoneinfo) is the authoritative record of every timezone rule ever published, updated multiple times a year. Operating systems, browsers, and language runtimes all ship a snapshot; stale tzdata on a long-lived device is a quiet source of date bugs after every DST or political change.

IANA tzdata / Olson database / zoneinfo: the same thing under different names
Each region is identified by an IANA name like America/Los_Angeles, Europe/Berlin, Asia/Kolkata
Releases are published several times a year; the version reads like 2026a, 2026b
Etc/GMT±N uses the POSIX sign convention — Etc/GMT-8 is UTC+8 (the sign is reversed)
Recent political changes: Kazakhstan unified on UTC+5 (2024), Mexico abolished DST (2022), Greenland moved to UTC-2 (2023), British Columbia adopted permanent UTC-7 (2026)
EU twice-yearly DST transitions are governed by IANA rules, not by individual countries
Devices with old tzdata produce silently-wrong local times after any DST or zone change — keep tzdata current

FAQ

What is the difference between UTC and GMT?: UTC (Coordinated Universal Time) and GMT (Greenwich Mean Time) are nearly identical for everyday use: both sit at the zero meridian with no UTC offset. The technical difference is that GMT is a timezone based on mean solar time at the Royal Observatory in Greenwich, while UTC is an atomic time standard. Most programming contexts use UTC.
How does daylight saving time affect Unix timestamps?: Unix timestamps are not affected by daylight saving time because they are always measured in UTC seconds. DST only affects the display of a timestamp when it is converted to a local timezone. This converter applies the correct DST offset for the selected timezone and date automatically.
What is an IANA timezone identifier?: An IANA timezone identifier (also called an Olson timezone name) is a string like 'America/New_York' or 'Asia/Tokyo' that uniquely identifies a timezone region and its full history of UTC offsets and DST rules. They are maintained in the IANA Time Zone Database and supported by all major operating systems and programming languages.
Should I store UTC or local time in my database?: Always store UTC (or Unix timestamps) in your database. Store the user's timezone preference separately and convert to local time only at display time. This avoids ambiguity during DST transitions and makes cross-timezone comparisons reliable.