Time Zones and Daylight Saving Time: A Plain Guide

By Wonho Lee. Updated October 8, 2026.

Anyone who has scheduled a call between two countries knows how easy it is to be an hour off. Time zones and daylight saving time look simple until two places change their clocks on different days. This guide explains the basics in plain language, points out the traps, and shows how to avoid them when you plan meetings, travel or live events.

UTC and offsets

UTC (Coordinated Universal Time) is the world's common reference. It does not change with the seasons. Every time zone is described as an offset from UTC. New York in winter is UTC-5, London in winter is UTC+0, and Tokyo is UTC+9 all year. When it is 12:00 in UTC, it is 07:00 in New York in winter and 21:00 in Tokyo. If you give someone a time together with its UTC offset, there is no ambiguity.

Not every zone is a whole hour

Most offsets are whole hours, but several are not. India uses UTC+5:30 all year. Nepal uses UTC+5:45. Newfoundland in Canada uses UTC-3:30 in standard time. China spans a very large area but uses a single time zone, UTC+8, for the whole country. When you convert times, do not assume that the minutes stay the same.

Why abbreviations are risky

Abbreviations such as EST, CST and IST feel official but are ambiguous. CST can mean Central Standard Time in North America, China Standard Time, or Cuba Standard Time. IST can mean India, Israel or Irish Standard Time. Computers avoid the problem by using names from the IANA time zone database, such as America/New_York, Europe/London or Asia/Kolkata. These names stand for a place and its whole history of rules, including daylight saving changes. The world clock on this site uses these names through your browser.

What daylight saving time does

Daylight saving time (DST) moves clocks forward, usually by one hour, for part of the year so that evenings are lighter. Many countries use it, but many do not. Within the United States, most states observe it, while Hawaii and most of Arizona do not. Countries near the equator usually have no reason to use it because day length changes little during the year.

• United States and Canada: clocks go forward on the second Sunday in March and back on the first Sunday in November, with some exceptions.
• European Union: clocks go forward on the last Sunday in March and back on the last Sunday in October, at the same moment across the member countries.
• Southern hemisphere: countries that use DST, such as New Zealand and parts of Australia, move the clocks in the opposite season, because their summer is in our winter.

Because the dates differ, the gap between two cities can change for a few weeks each year. New York and London are normally five hours apart, but for a short time in March, and again in late October, the gap is four hours, because the two places change on different dates.

The missing hour and the repeated hour

When clocks go forward, one hour does not exist in that place: 02:30 may never happen on that day. When clocks go back, one hour happens twice. This affects anything that depends on exact times, such as alarms, scheduled jobs, shifts and recurring events. A day with the clocks changed has 23 or 25 hours instead of 24. A countdown timer that measures an elapsed duration is not affected, but a timer that aims for a time of day on the clock can be.

Rules change

Governments sometimes change time zone rules, sometimes with only a few weeks of notice. Rely on devices and software that update their time zone data automatically, and do not hard-code dates from memory for important events. When you are planning something critical, check the date of the change for the specific places involved.

Scheduling across time zones: a short checklist

1. Decide the time in the time zone of the person who is least flexible, then convert it for everyone else.
2. Always write the date, the time and the time zone, for example "Thursday 14 March, 15:00 London time (UTC+0)".
3. Use city names rather than abbreviations.
4. For events near a clock change, check each location's date separately.
5. Use a calendar app that adjusts to each person's zone, or compare the cities on a world clock before you send the invitation.

Using the world clock

Open the Full Screen Clock and World Time, add the cities you care about, and compare their local times at a glance. The clock follows the time zone database in your browser, so daylight saving changes are handled automatically. Your city list is saved only on your own device. Need to count down to a meeting? Set a countdown timer for the remaining time.