Guides
Time Zone Conversion: Five Mistakes That Cost a Meeting
Offsets are not fixed, dates change more often than you expect, and daylight saving does not switch everywhere on the same day. How to convert times without getting it wrong.
Updated: 2026-10-19 · 6 min read
Converting a time between zones looks like adding or subtracting a fixed number of hours. It is not. The offset depends on the date, the region, and sometimes on a government decision made a few weeks earlier.
Offsets are not fixed
Every zone has a standard offset and, in many countries, a daylight saving offset that applies for part of the year. New York is UTC−5 in winter and UTC−4 in summer. London is UTC+0 and UTC+1. Sydney is UTC+10 and UTC+11.
That means a conversion is only correct for a specific date. “New York is five hours behind London” is true in January and wrong in July, when it is four hours behind.
Convert through UTC, not directly
The reliable method is two steps: convert the source time to UTC, then convert UTC to the target zone. Adding the difference between two local offsets works only when both zones are on the same side of a daylight saving change.
09:00 in Shanghai (UTC+8) is 01:00 UTC. New York in summer is UTC−4, so the same instant is 21:00 on the previous day. Doing it in one step — “Shanghai is twelve hours ahead of New York” — happens to work here, but fails in the weeks when one zone has changed and the other has not.
The date changes more often than you think
Any conversion that crosses midnight changes the calendar date, and the change can go either direction. A morning meeting in Tokyo is the previous evening in London; an evening call in Los Angeles is the next morning in Shanghai.
The practical consequence is that “same day” is not a property of a meeting, it is a property of the observer. When you send an invitation, always include the date in each participant's zone rather than assuming they will work it out.
Daylight saving gaps and overlaps
When clocks go forward, one hour of local time does not exist. A time set inside that gap — 02:30 on a spring-forward night in most of Europe — is not a valid local time. When clocks go back, one hour occurs twice, and 02:30 is ambiguous.
Scheduling software usually resolves the gap by moving forward and the overlap by choosing the first occurrence, but not all of it agrees. For anything critical, avoid the two hours around the change entirely.
The northern and southern hemispheres switch in opposite directions and on different dates, and some regions have abolished daylight saving while neighbours keep it. Two zones switching “in March” can still be three weeks apart.
The date line and the 24-hour trap
Crossing the international date line can move a time by a full day without changing the clock. Auckland at 09:00 on Monday is Los Angeles at 13:00 on Sunday — a 21-hour difference that includes a date change in the opposite direction from what most people expect.
If a difference comes out larger than 14 hours or smaller than −12, you have probably crossed the date line and should check the date, not just the clock.