Time Zone Converter

Convert a date+time from one IANA time zone to another.

Inputs

Result

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How to use this calculator

  • Enter date and time (24-hour HH:MM).
  • Pick source and target time zones.
  • Read converted time and offset difference.

About this calculator

Converts a wall-clock time in one IANA time zone to the equivalent wall-clock in another. Handles daylight saving time correctly via the Intl.DateTimeFormat API. The "hour difference" depends on date — DST shifts affect EU and US transitions in different weeks.

What this calculator does

This calculator converts a wall-clock date and time from one IANA time zone to another, handling daylight-saving transitions correctly via the browser's Intl.DateTimeFormat engine (which uses the system's IANA tz database snapshot). It surfaces the source UTC offset, the canonical UTC instant, the target UTC offset, and the absolute hour difference at that specific date — which matters because the difference between New York and London is 5 hours in winter but 4 in spring (US enters DST before the EU does).

How it works — the formula

Target_local = Source_local + (Target_offset_from_UTC − Source_offset_from_UTC) DST adjustments add or subtract 1 hour during the appropriate window.

A time zone is an offset from Coordinated Universal Time (UTC) plus optional daylight-saving rules. The IANA Time Zone Database (tz database) — maintained collaboratively and distributed via ICANN — is the authoritative source most operating systems use; it captures both current offsets and historical changes (e.g. Europe/Moscow has flipped DST policy three times this century). Subtract the source offset to reach UTC, then add the target offset to reach the destination local time; daylight-saving boundaries (US: 2nd Sunday March / 1st Sunday November; EU: last Sunday March / last Sunday October) introduce 1-hour discontinuities.

Sources: IANA Time Zone Database (tz database) · NIST — Time Service Department (UTC dissemination) · IETF RFC 9557 / RFC 6557 — Procedures for maintaining the tz database

Worked examples

Example 1
EST → JST
Inputs:
9:00 am New York (UTC−5) → Tokyo (UTC+9)
Output:
11:00 pm same day Tokyo (offset diff = +14 hr)

Winter-DST case. The 14-hour gap shrinks to 13 hours when New York is on DST (March–November) because Japan does not observe DST.

Example 2
DST boundary — non-existent local time
Inputs:
2026-03-08 2:30 am New York
Output:
Does not exist — clocks jump from 2:00 to 3:00 in the US "spring forward" transition

Calendaring systems handle this by either bumping to 3:00 or rejecting the input. ISO 8601 recommends always recording instants in UTC for events near a DST boundary.

Example 3
Half-hour offset
Inputs:
India Standard Time = UTC+5:30
Output:
9:00 am UTC = 2:30 pm IST (some zones use 30- and 45-minute offsets)

Non-hour offsets exist: India (+5:30), Nepal (+5:45), Newfoundland (−3:30), Chatham Islands (+12:45). Simple "subtract N hours" mental math breaks for these.

When to use this vs other tools

Time Zone Converter handles wall-clock conversion. For date arithmetic, business-day counting, or single-zone calendar work, reach for a more specific tool.

  • Days Between Dates

    Use when the units are dates, not date+time. Both endpoints are interpreted as UTC midnight so the count is time-zone-independent — exactly what you want for deadlines and trip durations.

  • Business Days Calculator

    Use when the result should exclude weekends and US federal holidays — useful for SLAs and procurement timelines that operate in a single time zone's business calendar.

  • Date Add / Subtract

    Use to add or subtract days, weeks, months, or years from a known date — a different problem from converting the same instant between zones.

  • Day of Week

    Use to identify what day of the week a specific date falls on. Useful in conjunction with this tool to confirm that a converted-and-shifted date does (or does not) cross into a different weekday.

Authority note

Internet Assigned Numbers Authority (IANA)
IANA Time Zone Database (tz database) — the authoritative source for time-zone rules used by every modern operating system, maintained per IETF RFC 6557.

The IANA tz database (formerly Olson database) is the reference data set every modern programming language, OS, and browser ships. This calculator reads the database snapshot bundled with the user's browser via Intl.DateTimeFormat — so its answers match what any IANA-aware system on the same OS version would compute.

Limitations

  • Historical conversions before the IANA database's coverage start may be inaccurate; political changes are added as they happen.
  • The "spring forward" hour does not exist in local time; the "fall back" hour repeats. Schedule events with explicit UTC if either case matters.
  • Some zones use non-hour offsets (UTC+5:30, UTC+8:45) that simple "hour difference" tools handle incorrectly.
  • The IANA database expects standard updates (typically 5–10 releases per year); software with a stale database can be wrong by an hour for affected regions.

Time-zone calculations follow the live IANA database snapshot bundled with the running browser/OS. For business-critical scheduling near DST boundaries, double-check with the destination party.

Frequently asked

Why are the offsets sometimes different across the year?+
Daylight saving time. NY-to-London is 5h in winter, 4h in spring (NY DST starts before London), 5h again summer, 4h again fall.
What's a "wall-clock" time?+
The time you'd see on a clock in a given time zone. "14:30 New York time" means whatever UTC moment a New York clock reads 14:30.
Why use IANA names like "America/New_York"?+
IANA names are unambiguous and DST-aware. "EST" alone is ambiguous (Eastern Standard or Eastern Summer?).
How accurate is this?+
Very — uses standard JavaScript Intl API, which uses the system's tzdata. Should match any modern OS.
What if my time zone isn't listed?+
The calculator covers 10 common zones. For others, the underlying logic supports any IANA name; future versions may add a custom-input field.

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