Time Zone Converter
Convert any time across global time zones using the Intl API.
About the Time Zone Converter
This Timezone Converter translates a date and time from one IANA timezone to another, correctly handling Daylight Saving Time transitions. It uses the browser’s built-in Intl.DateTimeFormat API with the timeZone option, which is populated from the Unicode CLDR (Common Locale Data Repository) — the same source used by Java, .NET, and PHP’s DateTimeZone class.
Timezones are surprisingly complex. The United States has 6 standard time zones (Eastern, Central, Mountain, Pacific, Alaska, Hawaii), but the actual count is higher because some states do not observe DST (Arizona, Hawaii) and some are split (Florida, Texas, Indiana, Tennessee, Kentucky, North Dakota, South Dakota, Nebraska, Kansas, Texas, Idaho, Oregon). Internationally, the IANA timezone database lists 397 named zones covering every observed local time rule, including historical changes (the USSR switched to year-round DST in 1991, North Korea created its own timezone in 2018, Venezuela shifted 30 minutes in 2007 and back in 2016).
The conversion math is: utc = source_local - source_offset; target_local = utc + target_offset. The offsets account for DST — if the source timezone is currently observing DST, the offset is one hour ahead of standard time. The tool computes offsets dynamically using Intl.DateTimeFormat with the shortOffset option, so DST transitions are handled automatically.
How It Works
The browser’s Intl.DateTimeFormat API, defined in the ECMAScript Internationalization API Specification (ECMA-402), provides timezone-aware date formatting. The timeZone option accepts an IANA timezone identifier (e.g. America/New_York) and renders the date in that zone, accounting for DST rules.
The conversion algorithm has three steps:
1. The datetime-local input string is parsed as a wall-clock time in the user’s local timezone. This gives us a JavaScript Date object that represents ‘that wall-clock time, in the user’s timezone’.
2. We compute the offset between the source timezone and the user’s local timezone by formatting the same instant in both zones and subtracting. This gives us a millisecond offset.
3. We add the offset to the original Date, producing a new Date object that represents the same wall-clock time in the source timezone (interpreted as UTC).
The final step is to format the resulting UTC instant in the target timezone. The Intl.DateTimeFormat API handles this directly: passing timeZone: 'Asia/Tokyo' formats the instant as a Tokyo wall-clock time, including DST adjustments (Tokyo does not observe DST, but the API handles it correctly for zones that do).
The IANA timezone database is updated several times per year as governments change their DST rules. The browser vendor bundles the database and ships updates with security patches; users on out-of-date browsers may see incorrect results for recently-changed zones (e.g. Mexico abolished DST in 2022, Egypt reinstated it in 2023).
Worked Examples
Convert 2025-03-15 09:00 from America/New_York to Asia/Tokyo: New York on March 15, 2025 is in EDT (UTC-4, since DST starts in March). Tokyo is JST (UTC+9, no DST). The conversion: 09:00 EDT = 13:00 UTC = 22:00 JST. So the target time is 2025-03-15 22:00 JST.
Convert 2025-12-25 14:00 from Europe/London to Asia/Kolkata: London in December is in GMT (UTC+0, no DST). Kolkata is IST (UTC+5:30, no DST). The conversion: 14:00 GMT = 14:00 UTC = 19:30 IST. Note the half-hour offset — India is one of several countries (Iran, Myanmar, parts of Australia) with non-integer-hour offsets.
Convert 2025-07-04 18:00 from America/Los_Angeles to Pacific/Auckland: Los Angeles in July is in PDT (UTC-7, DST in effect). Auckland in July is in NZST (UTC+12, no DST in southern-hemisphere winter). The conversion: 18:00 PDT = 01:00 UTC (next day) = 13:00 NZST (next day). The target is 2025-07-05 13:00 NZST — note the date rollover.
For a meeting across three timezones (New York, London, Tokyo), the convenient overlap is 9 AM New York / 2 PM London / 11 PM Tokyo — the only 1-hour window when all three are in business hours on the same day.
When to Use This Tool
- Scheduling international conference calls — find a time that is reasonable for participants in multiple zones.
- Coordinating product launches across regions — ensure the launch is simultaneous globally (or staggered for marketing reasons).
- Flight booking — understand the local arrival time when crossing timezones (jet lag is partly a function of arrival time).
- Webinar and live-stream scheduling — show start times in multiple zones on the registration page.
- Software deployment windows — coordinate a release window that minimizes impact across regions.
- Customer support coverage — ensure 24/7 coverage by staggering shifts across geographies.
- Legal deadline tracking — court filing deadlines are often expressed in the court’s local time, which may differ from your own.
Limitations & Disclaimer
This converter uses the browser’s built-in Intl.DateTimeFormat API, which is populated from the Unicode CLDR and IANA timezone databases. Browser vendors update these databases with security patches; users on out-of-date browsers may see incorrect conversions for recently-changed zones (Mexico abolished DST in 2022, Egypt reinstated it in 2023). DST transition gap and overlap times are interpreted per the ECMAScript spec (gap: post-transition offset; overlap: first occurrence). The 18 zones listed cover the major business centers; for other zones, the converter falls back to a server-side calculation. See our disclaimer for full terms.
Frequently Asked Questions
How does the converter handle Daylight Saving Time?
Automatically. The browser’s <code>Intl.DateTimeFormat</code> API uses the IANA timezone database, which records the exact DST rules for every zone, including historical changes. The converter checks whether DST is in effect at the source and target times and applies the correct offset. For example, New York is UTC-5 in winter (EST) and UTC-4 in summer (EDT).
What is the IANA timezone database?
The IANA Time Zone Database (also called tz database or zoneinfo) is the canonical source of timezone rules, maintained by Paul Eggert and a community of contributors since 1986. It records every observed timezone rule change since 1970 (and many historical changes before that). The database is updated 4–10 times per year as governments change their DST rules. See <a href='https://www.iana.org/time-zones'>iana.org/time-zones</a>.
Why is India UTC+5:30 and not UTC+5 or UTC+6?
India uses a half-hour offset (UTC+5:30) for historical reasons — the offset was set during British colonial rule to align the clocks with the local solar time at the Allahabad Observatory. Several other zones also use half-hour or 45-minute offsets: Iran (UTC+3:30), Myanmar (UTC+6:30), Nepal (UTC+5:45), and parts of Australia (UTC+9:30).
What happens when a timezone changes its rules?
The browser vendor bundles the IANA database and ships updates with security patches. If your browser is out of date (more than 6 months old), you may see incorrect conversions for recently-changed zones. Modern browsers auto-update within days of a release. For server-side applications, regularly update the OS timezone package (e.g. <code>tzdata</code> on Linux, <code>cldr</code> on macOS).
Can I convert a time that falls in a DST transition gap?
In most cases, yes — the converter applies the post-transition offset. The ‘spring forward’ transition in March (US/Europe) creates a one-hour gap (e.g. 2:00 AM to 3:00 AM does not exist); the converter interprets such times as if they were after the transition. The ‘fall back’ transition in November creates a one-hour overlap (e.g. 1:30 AM occurs twice); the converter uses the first occurrence (DST in effect).
How do I find my own timezone?
Run <code>Intl.DateTimeFormat().resolvedOptions().timeZone</code> in your browser’s JavaScript console — it returns the IANA identifier (e.g. <code>America/New_York</code>). This is the timezone your browser uses for date pickers and time displays.
Last updated: September 9, 2026 · Author: HT99 Tools Editorial Team