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Time Zone Converter

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Time Zone Conversion

Convert a wall-clock time from one IANA zone to one or more target zones. Offsets shift seasonally for zones that observe daylight saving time.

Target time = Source time + (Target offset − Source offset)

How It Works

The time zone converter anchors your source date and time to a single UTC instant using the IANA zone database, then renders that instant in each target zone via Intl.DateTimeFormat. Because each zone's UTC offset is looked up dynamically, the math is correct across spring-forward and fall-back transitions — for example, 10:00 in New York lands on 07:00 in Los Angeles in July (both observing DST, 3 hours apart) but on 07:00 in Los Angeles in January (both on standard time, still 3 hours apart). Zones that don't observe DST (UTC, Japan, India, Arizona) keep a fixed offset year-round.

Example Problem

Convert 10:00 on April 15, 2026 in America/New_York to America/Los_Angeles, UTC, and Asia/Tokyo.

  1. Identify the source: wall-clock 10:00 on 2026-04-15 in America/New_York.
  2. In mid-April, New York observes Eastern Daylight Time (EDT), which is UTC−04:00.
  3. Anchor the source to UTC: 10:00 − (−04:00) = 14:00 UTC on 2026-04-15.
  4. Render 14:00 UTC in Los Angeles (PDT, UTC−07:00): 14:00 − 07:00 = 07:00 on 2026-04-15.
  5. Render 14:00 UTC in UTC: 14:00 on 2026-04-15.
  6. Render 14:00 UTC in Tokyo (JST, UTC+09:00, no DST): 14:00 + 09:00 = 23:00 on 2026-04-15.

All four wall-clock answers correspond to the same physical instant. If the conversion crosses a DST transition or the international date line, the local date can roll forward or back by one calendar day.

Key Concepts

Time zones are offsets from Coordinated Universal Time (UTC), the modern successor to Greenwich Mean Time (GMT). The IANA Time Zone Database (tz database) is the authoritative source — zones like America/New_York encode both the current rules and the historical changes for that location. Standard short codes like EST, EDT, PST, JST, BST refer to specific zone-and-season pairs (Eastern Standard Time vs Eastern Daylight Time), so when scheduling across zones it is safer to specify the IANA name than the abbreviation. Daylight saving time, where observed, shifts the offset by one hour for part of the year — in the US that is the second Sunday of March through the first Sunday of November.

Applications

  • Scheduling international meetings — pick a time that works during business hours in every participant's zone
  • Travel planning — convert flight departure and arrival times between origin and destination zones
  • Server logs — translate UTC timestamps into your local zone for analysis
  • Remote work — coordinate handoffs across team members in different cities
  • Sports and live events — convert game start times announced in the home venue's zone
  • Financial markets — match opening and closing bells in New York, London, and Tokyo against your local schedule

Common Mistakes

  • Confusing EST with ET — “ET” is the umbrella term that switches between EST (winter) and EDT (summer); EST specifically means standard time year-round and is wrong half the year
  • Forgetting that not every region observes DST — Arizona, Hawaii, most of Asia, and most of Africa do not, so the offset to those zones changes when the rest of the country springs forward or falls back
  • Assuming GMT is the same as UTC — GMT is a time zone (used in the UK, with BST in summer); UTC is an atomic time standard. They are usually equal but conceptually distinct
  • Treating offset arithmetic as 24-hour-modular only — a time conversion can roll the date forward or backward, especially across the international date line (e.g., LA evening → Tokyo next-morning)
  • Using the wrong source zone — entering 10:00 in “your local time” when you actually meant 10:00 in the meeting host's time zone produces a result that's off by the difference between the two

Frequently Asked Questions

How do I convert between time zones?

Subtract the source zone's UTC offset from the source wall-clock time to get a UTC instant, then add the target zone's UTC offset to get the target wall-clock time. For example, 10:00 New York (EDT, UTC−04:00) is 14:00 UTC, which is 07:00 Los Angeles (PDT, UTC−07:00). Use the IANA zone names — they automatically handle daylight saving time transitions.

What is daylight saving time (DST)?

Daylight saving time shifts a zone's UTC offset forward by one hour for part of the year, typically March through November in the Northern Hemisphere. The intent is to shift more usable daylight to the evening. Not every country or even every region within a country observes DST — Arizona, Hawaii, most of Asia, and most of Africa do not. When DST is active, that zone is on its “daylight” variant (EDT, PDT, BST, CEST, AEDT); when DST is inactive, it is on its “standard” variant (EST, PST, GMT, CET, AEST).

EST vs EDT — what is the difference?

EST is Eastern Standard Time (UTC−05:00), used in the US Eastern zone from early November through mid-March. EDT is Eastern Daylight Time (UTC−04:00), used from mid-March through early November. “ET” is the umbrella that switches between the two automatically. When people say “3pm ET” for a meeting in July, they mean 3pm EDT — the offset is UTC−04:00, not UTC−05:00. This calculator uses the IANA America/New_York zone so the right variant is picked automatically based on the date.

Why does the offset change between summer and winter?

Because most zones in the US and Europe observe daylight saving time. From mid-March to early November in the Northern Hemisphere, those zones move their offset forward by one hour. So New York is UTC−05:00 in January but UTC−04:00 in July — same zone, different offset. The Southern Hemisphere (e.g., Sydney, Auckland) does the opposite: DST runs roughly October through April. Always specify the date when converting times — “10am New York” is ambiguous unless you know which side of the transition it falls on.

How do I pick a meeting time that works for international teams?

Use a UTC anchor. Pick a UTC time that lands in business hours (roughly 09:00–18:00 local) for the largest number of participants, then convert to each person's zone. Tools that show the working-hours overlap visually are helpful, but the math is the same as this converter: shift the UTC anchor by each zone's offset and check the resulting local hour. Common compromise windows for US-Europe meetings are 13:00–16:00 UTC (morning in the Americas, afternoon in Europe).

Daylight saving time start and end dates by country

United States and Canada: DST runs from the second Sunday of March through the first Sunday of November (Arizona, Hawaii, and most of Saskatchewan do not observe). European Union and UK: DST (called “summer time”) runs from the last Sunday of March through the last Sunday of October. Australia: DST runs from the first Sunday of October through the first Sunday of April (only NSW, VIC, SA, TAS, ACT observe — QLD, NT, WA do not). New Zealand: late September through early April. Most of Asia, Africa, and South America do not observe DST.

What is UTC and how is it different from GMT?

Coordinated Universal Time (UTC) is the modern time standard maintained by atomic clocks, with leap seconds occasionally inserted to keep it within 0.9 seconds of mean solar time at the Royal Observatory in Greenwich. Greenwich Mean Time (GMT) is the historical name for that solar mean time and is also the legal civil time in the UK during winter (in summer the UK switches to BST, British Summer Time, UTC+01:00). For most practical purposes UTC and GMT are interchangeable, but technically UTC is the atomic standard and GMT is a time zone.

Can the calculator handle the international date line?

Yes. When you convert a time from a zone west of the date line (like Los Angeles, UTC−08:00) to a zone east of it (like Tokyo, UTC+09:00), the result naturally rolls to the next calendar day if the underlying UTC instant lands there. For example, 22:00 on April 14 in Los Angeles is 14:00 on April 15 in Tokyo — same physical moment, but Tokyo's wall clock shows the next day.

Reference: Time zone data follows the IANA Time Zone Database (tz database) as exposed by ECMAScript Intl.DateTimeFormat. Daylight saving rules update automatically with the host platform's tz data.

Time Zone Conversion Formula

The conversion anchors the source wall-clock to a single UTC instant, then renders that instant in each target zone:

UTC = Source − Source_offset · Target = UTC + Target_offset

Where:

  • Source — the wall-clock date and time in the source zone
  • Source_offset / Target_offset — the UTC offset of each zone on that specific date (e.g., UTC−04:00 for New York in July, UTC−05:00 for New York in January)
  • UTC — the unambiguous instant that both wall-clock times correspond to

The IANA tz database holds the historical and current DST rules for every named zone, so the offset is selected based on the date being converted.

Worked Examples

International Meeting

What time is 10:00 AM New York in London, Tokyo, and Sydney on April 15, 2026?

A team in New York wants to schedule a meeting at 10:00 AM local time and needs to know the corresponding times in London, Tokyo, and Sydney.

  • Source: 10:00 on 2026-04-15 in America/New_York (EDT, UTC−04:00)
  • UTC anchor: 10:00 + 04:00 = 14:00 UTC on 2026-04-15
  • London (BST in mid-April, UTC+01:00): 14:00 + 01:00 = 15:00
  • Tokyo (JST, UTC+09:00, no DST): 14:00 + 09:00 = 23:00
  • Sydney (AEST in mid-April, UTC+10:00 — DST ends early April): 14:00 + 10:00 = 00:00 on April 16

10:00 New York = 15:00 London = 23:00 Tokyo = 00:00 next-day Sydney.

Flight Arrival

A flight leaves Los Angeles at 23:30 on December 1 and lands in Tokyo 11 hours later — what is the local arrival time?

Convert the LA departure to UTC, add the flight duration, then convert to Tokyo local time. December is winter in LA (PST, UTC−08:00) and Tokyo never observes DST.

  • Departure: 23:30 on 2025-12-01 in America/Los_Angeles (PST, UTC−08:00)
  • UTC departure: 23:30 + 08:00 = 07:30 UTC on 2025-12-02
  • Add 11 hours of flight: 18:30 UTC on 2025-12-02
  • Tokyo arrival (JST, UTC+09:00): 18:30 + 09:00 = 03:30 on 2025-12-03

Tokyo local arrival is 03:30 (early morning) on December 3, two calendar days after departure.

DST Transition

How does the New York → London offset change between January and July?

Both New York and London observe DST, but on slightly different dates. The offset between them is usually 5 hours, but can briefly be 4 hours in March and again in November when only one of the two zones has shifted.

  • January: New York is EST (UTC−05:00), London is GMT (UTC+00:00). Offset = 5 hours.
  • July: New York is EDT (UTC−04:00), London is BST (UTC+01:00). Offset = 5 hours (both shifted by the same amount).
  • Late March: US has sprung forward (mid-March) but UK has not (late March). Offset briefly = 4 hours.
  • Late October / early November: UK falls back (late October) before US (early November). Offset briefly = 4 hours.

for ~3 weeks per year, New York is only 4 hours behind London instead of the usual 5. Always specify the date when scheduling across DST transitions.

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