How to Convert Time Zones Correctly (Including DST)

The correct way to convert a time between time zones, why a fixed offset table gets it wrong twice a year, and how to handle day-crossing conversions.

Converting a time between cities looks like simple addition or subtraction, but two things trip people up constantly: daylight saving time, and conversions that cross midnight into a different calendar day.

Why a fixed offset is wrong half the year

Most major cities don't sit at a constant distance from UTC — they shift by an hour for daylight saving during part of the year, and different countries change on different dates. New York is UTC−5 in January and UTC−4 in July; assuming a single fixed offset for "New York time" gets half the calendar wrong. The only reliable method is looking up the real offset for the specific date you're converting, not a static table.

Why the date can change

Once two cities are more than a few hours apart, a late-evening time in one often lands on the next calendar day somewhere far enough ahead (or the previous day somewhere far enough behind). 11:00 PM in Los Angeles is already 3:00 PM the next day in Tokyo — always check the date, not just the clock time, when converting across a large offset.

The fix

Use a converter that reads each city's real time zone rules for your specific date (the IANA time zone database, which browsers and operating systems already maintain) rather than a hand-built offset table. The Time Zone Converter does exactly this, and flags any date change directly in the result.