Tag Archives: time zones

About daylight saving time

Daylight saving time is a common practice in large swaths of the Western world. I’ve heard many misunderstandings about it, so I thought I’d try to set some straight.

Name

In North America, we call this concept daylight saving time, often rendered as daylight savings time. In most other English-speaking countries, this is called summer time.

As indicated by the non-North American name, DST refers specifically to the summer portion of the year; the winter portion of the year is merely standard time.

What’s the point of DST?

Originally, daylight saving time/summer time was devised to give us more sunlight on summer evenings.

Most of the purposes of DST are obsolete now, so many advocate for getting rid of it altogether.

Am I using EST?

If it’s summer, and you live on the East Coast of the US, you are not using EST. EST stands for “Eastern Standard Time,” which is a term specifically to distinguish it from its summertime equivalent: “Eastern Daylight Time” (EDT). So it is incorrect to say something like “11am EST on August 14th”… unless you actually mean noon in New York time on that day. If you want a term that you can use safely year-round, just use “Eastern Time” (ET).

The nomenclature is equivalent in the other US time zones: Los Angeles’ PST (Pacific Standard Time) switches to PDT in the summer, just as Denver’s MST does to MDT, and Chicago’s CST to CDT. And the terms PT, MT, and CT are acceptable regardless of daylight saving time status.

In Britain, people make the similar mistake of saying GMT (Greenwich Mean Time, their equivalent of EST) year-round. During summer, Britain does not use GMT at all; they use British Summer Time (BST).

To hit the point home, here are the current times in four different timezones (might not work on mobile devices):

Eastern Standard Time
Eastern Daylight Time
Greenwich Mean Time
British Summer Time

Complexities

DST observance map of the world
Blue: observes DST. Orange: formerly observed DST. Red: never observed DST.
Source: http://upload.wikimedia.org/wikipedia/commons/2/29/DaylightSaving-World-Subdivisions.png

Some countries use DST/summer time, and others don’t. Some have regions that do and regions that don’t (N.B. Arizona and Hawaii, 3 states in Australia, etc.). Some countries or regions used to use daylight saving time but no longer do. Others have changed the dates or times of switching the clocks.

One particularly confusing element is when coordinating between the northern hemispheres. So around March, when DST-observing countries set their clocks forward for summer time, DST-observing countries in the southern hemisphere are turning their clocks backward as their summer time is ending.

Example: São Paulo, Brazil uses GMT -3 hours as its standard time zone. New York City uses GMT -5 hours as its standard time zone. March through October is summer time in the northern hemisphere and winter in the southern hemisphere, so NYC uses Eastern Daylight Time, which is GMT -4 hours (while São Paulo is on standard time: GMT -3 hours). So NYC is 1 hour behind São Paulo. But November through February is winter in NYC and summer in São Paulo – so NYC is in Eastern Standard Time (GMT-5) while São Paulo is in daylight time (GMT-2), making them 3 hours apart.

What’s it look like during the overlap periods between the clock changes? A mess.

Things that DST doesn’t do

You may have grievances about daylight saving time, but there are some things that you just can’t blame on it, like recent droughts or your crops dying.

Let’s share what we learned about the magic of daylight savings time!