Time Zone Converter

Features

• Compare unlimited zones side by side
• Daylight saving handled automatically by the browser
• Working-hours highlighting for meeting planning
• Shows UTC offset and day difference for each zone
• Board persists on your device between visits

How to use it

1. Set the date and time, and the zone it is in.
2. Add the other zones you care about.
3. Green rows are within 9am to 6pm locally.
4. Copy the board to paste into a meeting invite.

Why timezone maths goes wrong

Offsets are not fixed properties of a place. New York is UTC−5 in winter and UTC−4 in summer, so storing “−5” alongside a timestamp gives the wrong answer for half the year. The correct approach is to store the instant in UTC and the location as an IANA zone identifier such as America/New_York, then convert at display time. That is what this tool does through the browser's own Intl implementation, which carries the full IANA database.

Daylight saving transitions do not happen simultaneously worldwide. Europe and North America change on different weekends, producing a two-week window each spring and autumn when the usual offset between London and New York is one hour different from normal. Recurring meetings scheduled across that boundary silently shift for one party, which is a common and irritating source of missed calls.

The southern hemisphere inverts everything, Australia's daylight saving runs during the northern winter, and a large part of the world, including most of Asia and Africa, does not observe it at all. India is offset by 5 hours 30 minutes, Nepal by 5 hours 45, and the Chatham Islands by 12 hours 45, which is why any code assuming whole-hour offsets eventually breaks.