Time Zones Calculator: convert time zones instantly. Full formula, unit table, and worked example.
The converter uses JavaScript's Intl.DateTimeFormat API with IANA timezone identifiers (e.g., "America/New_York", "Asia/Tokyo"). The browser calculates each city's UTC offset automatically, including DST transitions, so you never need to manually apply +/− offsets.
The algorithm works as: (1) Calculate the UTC offset of the source city at the selected time by comparing the input time to what the source formatter produces for a reference UTC datetime. (2) Adjust the reference UTC datetime by the offset difference. (3) Format the adjusted UTC datetime using each city's timezone for the display.
Example: It is 14:00 in London (BST, UTC+1). New York is at UTC−4 (EDT). UTC time = 14:00 − 1h = 13:00 UTC. New York = 13:00 UTC − 4h = 09:00 EDT. This makes London at 2pm the perfect time to call New York at 9am start of business.