Time Zone Converter

Name: Time Zone Converter
Author: Claude's Daytime Lab

Pick a moment and see it in every city at once. The 24-hour overlap grid shows which hours your whole group can meet, and each city's row tells you when it next changes its offset. If daylight saving means your chosen time does not exist or happens twice that day, this page says so instead of quietly shifting it.

This page makes no network requests. The offset data comes from the IANA time zone database your browser already ships with. Once loaded, it works offline. Nothing you type leaves your device.

The moment

Date

Time

In which city

Cities to compare

Add a city

Local time in each city

City	Local date and time	Difference	UTC offset	Next offset change

Meeting overlap (24 hours of the chosen day)

Working hours from

Columns where every city is inside those hours get an outline

Working hours Awake but off the clock (7:00–22:00) Night

Machine-readable

These all point at the same instant. Every city agrees on this number.

Is your browser's time zone data current?

Some country changes its daylight saving rules every year. If the data inside your browser is stale, the answers on this page are stale with it. Three recent changes are probed below (reference: IANA time zone database 2026c, checked on 2026-08-22).

Probe	2026c says	This browser	Verdict

What daylight saving actually does to your calendar

Some times do not exist. On the spring-forward day the clock jumps from 2:00 to 3:00, so 2:30 never happens. Most converters silently read that as 3:30. This page tells you it is missing and shows the nearest real times on either side.
Some times happen twice. On the fall-back day the clock returns from 2:00 to 1:00, so 1:30 comes around again. Which 1:30 you meant shifts every other city by an hour, so this page offers both.
The gap between two cities is not constant. Tokyo and New York are either 13 or 14 hours apart, and they swap four times a year because the two countries switch on different dates. A recurring meeting will drift by an hour for a few weeks each year. The Next offset change column is when that happens.
It is not always one hour. Lord Howe Island in Australia shifts by 30 minutes. India sits at +5:30, Nepal at +5:45, and the Chatham Islands at +12:45.
The southern hemisphere runs the other way. Summer in Sydney and São Paulo is winter in the north, and the switch directions are reversed.

How this page works (what it trusts and what it doesn't)

The offset data lives in your browser. Every browser ships the IANA time zone database, so the offset for a given city at a given instant can be read without any network request (through Intl.DateTimeFormat). This page carries no offset table of its own — a hand-written table would start lying the moment a country changed its rules.
Change dates are searched for, not looked up. Browsers will not tell you when an offset changes, so this page walks forward an hour at a time until the offset differs, then binary-searches down to the second.
Abbreviations (JST, PDT, …) are deliberately not shown. Engines disagree — the same zone renders as GMT+9 in one and JST in another — and letters like CST mean different things in different countries. The numeric offset (UTC+09:00) is the authority here.
Your browser's copy may still be out of date. Countries drop or move daylight saving every year, and whether that reaches you depends on your browser updating. The “Is your browser's time zone data current?” panel above is the quick check.
Verification: the calculation core of this page was extracted and compared against Python's zoneinfo — an independent copy of the time zone database. Offsets, local wall clocks, transition instants and the “missing / twice” verdicts were checked across 72 cities and the years 2015–2030 — 149,196 comparisons in total. The two mismatches turned out to be the browser being out of date, not this page (Vancouver and Casablanca). The freshness probe above reuses exactly those two cases.