Time difference between cities calculator

A time difference between cities calculator answers the everyday travel and calling question: if it is a given local hour in one city, what time is it in the other, and how many hours sit between those two clocks? You name both cities and supply each city's fixed UTC offset in hours—minus five for New York on standard time, plus nine for Tokyo, plus twelve for Auckland, minus ten for Honolulu—plus the local hour and optional minute in the first city. The solver returns the signed hour difference, both local clocks as HH:MM, the calendar-day shift after midnight wrap, a shortest-path wrap into a plus-or-minus twelve hour window, and a flag that marks when the two offsets sit more than half a day apart, which is the civil-offset signature of a date-line crossing. Offsets outside plus or minus fourteen hours are rejected.

How the hour gap and both local clocks are computed

Every civil city clock can be written as a fixed offset from Coordinated Universal Time once you already know the seasonal schedule you care about. New York on Eastern Standard Time is UTC minus five; Tokyo is UTC plus nine; Delhi is UTC plus five and a half; Adelaide is UTC plus nine and a half; Kathmandu is UTC plus five and three quarters.

This calculator never looks up those numbers for you: you type the city names as labels and the two offsets as hours, then you give the local hour—and optionally the minute—already showing on the first city's clock. The signed hour difference is always offset of city B minus offset of city A. That single subtraction is the number of hours you add to the first city's local time to obtain the second city's local time at the same instant.

Work proceeds in whole minutes so half-hour and quarter-hour zones stay exact: the difference is multiplied by sixty and rounded to the nearest minute, then added to hour times sixty plus minute. The resulting minute total is folded back into a twenty-four-hour clock. Whatever remains after removing complete days is the HH:MM in city B; the number of complete days removed is the calendar-day offset, which can be minus two through plus two at the real extremes of UTC minus fourteen and UTC plus fourteen.

When the two cities share an offset the clocks match and the day offset is zero. When they do not, the response names which city is ahead, prints both clocks, and lists the substitution steps so you can check the arithmetic by hand before you book a call or a meeting.

Day rollover across the International Date Line

The International Date Line is the civil seam near one hundred eighty degrees longitude where the calendar date jumps. Cities on the far-east side of that seam use large positive offsets such as UTC plus twelve in Auckland, UTC plus thirteen in Samoa, or UTC plus fourteen on the Line Islands. Cities on the far-west side use large negative offsets such as UTC minus ten in Honolulu, UTC minus eleven in American Samoa, or UTC minus twelve on Baker Island.

Subtracting those offsets can produce a gap larger than twelve hours and, at the extremes, larger than twenty-four hours, so adding the gap to a local clock can land not only on the previous or next calendar day but two days away. Classic pairs illustrate the contract. Fifteen hundred in New York at UTC minus five is zero five hundred the next calendar day in Tokyo at UTC plus nine, a plus fourteen hour gap that flags the date line and wraps to minus ten hours the other way.

Noon in Auckland at UTC plus twelve is fourteen hundred the previous calendar day in Honolulu at UTC minus ten, a minus twenty-two hour gap that wraps to plus two hours. Nine in the morning in Samoa at UTC plus thirteen is nine in the morning the previous day in American Samoa at UTC minus eleven: same clock face, twenty-four hours and one calendar day apart.

Use those fields when you need to tell a traveler whether to write Tuesday or Wednesday on a boarding pass, not only what the wall clock will read.