How time zone conversion works
Every local time is a fixed offset from Coordinated Universal Time (UTC), such as UTC+5 for Pakistan Standard Time or UTC−5 for US Eastern Standard Time. To convert, you first turn the local time at the source into a single moment in UTC by subtracting its offset, then add the target zone's offset. The complication is that many places change their offset twice a year for daylight saving time. New York is UTC−5 in winter and UTC−4 in summer, so 12:00 in New York on 1 July is 16:00 UTC, but on 1 January it is 17:00 UTC.
Where the offsets come from
This tool uses the time zone database built into your browser through the standard Intl API. It knows the historical and current rules for each named zone, such as America/New_York or Asia/Kolkata, so daylight-saving transitions are applied on the correct dates. Nothing is downloaded, and no request is made to any server. The result is therefore only as current as your browser's own copy of the database, which is updated with normal browser updates.
Tips for scheduling across zones
- Always state the zone with the time: "15:00 UTC" or "3 pm Eastern". "3 pm" alone causes missed meetings.
- Be careful around the weeks when the US and Europe change clocks. They switch on different dates, so the gap between London and New York is briefly one hour different from usual.
- Some zones use offsets that are not whole hours, such as India (UTC+5:30) and Nepal (UTC+5:45). The table shows the exact offset for each city.
- Many countries, including Pakistan, India, Japan and China, do not observe daylight saving, so their offset never changes.
Edge cases
When clocks go forward, an hour of local time does not exist, for example 02:30 on the spring change day in New York. The tool resolves such a time to the nearest valid moment. When clocks go back, an hour happens twice; the converter uses the first occurrence. The date and time field uses your device's own date picker. Your inputs are never stored or sent anywhere.