Reading a world clock when half the cities are on summer time

A world clock is at its most useful when you stop reading the digits and start reading the gaps. Once you know that your colleague in Chicago is seven hours behind, you barely look at the number any more. That habit is efficient for about ten months a year and actively misleading for the other two.

The gaps move, and not all at once

Roughly a third of the world's countries shift their clocks by an hour for part of the year. The rest do not. So on any clock that mixes both kinds of city, some rows move in March and October and others never move at all. During the weeks when one region has changed and another has not, the difference between them is temporarily unlike anything you have memorised.

Two patterns cause most of the confusion:

• North America and Europe change on different weekends. North America springs forward earlier in March and falls back later in autumn, so there are gaps of a week or more each year when the usual Atlantic difference is an hour off.
• The southern hemisphere changes in the opposite direction. Australia and New Zealand are gaining daylight as Europe loses it. Across a full year the gap between London and Sydney moves through a two-hour range.

Which rows to distrust

In practice you only need to be suspicious of a handful of your cities, and you can work out which ones once and remember it. Cities in most of Asia, Africa and South America keep the same offset all year, so their rows are stable. Cities in Europe, North America, Australia, New Zealand and a few others move. If a row you rely on belongs to the second group, treat it as an approximation in March, April, September, October and November.

There are also places that observe daylight saving on a schedule tied to something other than the season, most notably where the change is aligned with Ramadan and therefore moves through the calendar year. If your clock includes one of those, it is worth reading the actual digits rather than trusting the gap at any time of year.

Read the offset, not just the time

The quickest way to tell whether a city has changed is to look at its offset from UTC rather than its clock reading. A city on UTC+1 in winter shows UTC+2 in summer, and that label changes on the day the clocks do. When two cities you compare regularly both show their summer offsets, your remembered gap is right again. When one has moved and the other has not, it is not.

What a live clock protects you from

A world clock computed live from the IANA time zone database, which is what the world clock here does, is correct on every one of these days without you knowing the rules. It reads each city's current offset for the moment you are looking, including the awkward weeks. The only thing it cannot do is stop you overriding it with a number you memorised in July, which is the real source of missed calls in late October.