How to Calculate the Time Difference Between Two Cities
The correct way to work out an hour gap using UTC offsets, with worked examples and the traps that break manual calculations.
The method: compare UTC offsets
Every timezone is defined as an offset from UTC, the global reference time. To find the difference between two cities, take each city's current UTC offset and subtract one from the other. Tokyo is UTC+9 all year. New York is UTC-5 in winter and UTC-4 during daylight saving. So in July the gap is 9 − (−4) = 13 hours, and in January it is 9 − (−5) = 14 hours — Tokyo ahead in both cases.
To convert a specific clock time, add the difference going toward the city that is ahead and subtract going toward the city that is behind. 9:00 AM in New York in July becomes 10:00 PM the same day in Tokyo. If the result passes midnight, the date changes too — which is exactly the detail manual calculations tend to drop.
Trap one: offsets are not always whole hours
India runs on UTC+5:30, Iran on UTC+3:30, Nepal on UTC+5:45, and parts of Australia use half-hour offsets as well. A colleague in Mumbai is 3 hours 30 minutes behind Seoul, not 3 or 4. Half-hour zones are where "roughly N hours apart" reasoning quietly produces meetings that start thirty minutes wrong.
There is no shortcut here other than using the real offset. If any city in your calculation is in South Asia, the Middle East, or central Australia, check the exact offset rather than rounding.
Trap two: the difference itself changes during the year
Daylight saving means a city pair can have two or even three different gaps across a year. London and Sydney illustrate the extreme: both cities shift their clocks, but in opposite seasons, so the gap cycles between 9, 10, and 11 hours. A conversion you memorized in March can be wrong in April without either of you moving anywhere.
The reliable habit is to treat any remembered offset as perishable. For a one-off meeting, look the difference up on the day you schedule it. For recurring meetings, re-check in mid-March and late October, when the US and Europe make their transitions on different weekends.
Trap three: crossing the date line
For pairs like Sydney and Los Angeles, the question is less "what hour" and more "what day". Sydney is far enough ahead that its Monday morning is still Sunday afternoon in California. Flights make this vivid: you can land in the Americas before your departure time, or lose a calendar day flying west.
When a conversion crosses midnight, always state the date alongside the time — "Tuesday 9:00 AM Sydney, Monday 3:00 PM Los Angeles". Writing both halves out is the only notation that survives forwarding to a third person in yet another timezone.