GMT vs. UTC, a daylight saving story

A few days ago was the last Sunday of October for this year, so we turned our watches one hour back. In many countries of the world, every last Sunday of March and October is when daylight saving time is usually activated and deactivated respectively. That is what makes the difference between GMT and UTC.

The short answer to what is the difference between GMT (Greenwich Mean Time) and UTC (Universal Time Coordinated) is that GMT changes with the daylight saving time, whereas UTC remains unchanged.

If you think that most countries of the world use daylight saving time, you are wrong. Programmers should be encouraged to base their programs on UTC, rather than GMT, because that is what is stable as far as Earth time is concerned.

To make it even clearer, in the winter time (November to March) GMT is exactly the same as UTC. In summer time (April to October) GMT is UTC plus one hour.


Lets use an example. Lets think of Athens, Greece who's timezone is GMT plus two hours. In the winter time, Athens is GMT + 2 hours and also UTC + 2 hours. In the summer time though, Athens is GMT + 2 hours, just as it was in the winter time, but it is UTC + 3 hours. That is because United Kingdom (the country that GMT originates from) conforms with the daylight saving time standard, so GMT time is increased by 1 hour. So, time between Athens and London is always 2 hours away, but Athens is not always 2 hours away from countries that do not use the daylight saving time like China, India, Russia, most of Australia e.t.c.

Why so much fuss for such a little thing like GMT and UTC?

The fuss not for everyone. It hassles people who make applications (software or otherwise) that address international audiences.

For example, if you plan to create a forum software for people all over the world, and you want them to see posts in their local time, you have to base your software in a universal standard. If you save the times of posts in UTC time and know what country the user is from, you can easily adjust the time to your user's local time.

Another example, similar to the previous one, would be to have authors who want to see when they got reviews for their products, or blog-posts. Again, they want to see the times of the reviews in their local time.

That is how standards help you put order to an otherwise chaotic input you may get from an international audience.

Useful links

While researching for this article I stumbled upon some useful links.

More information about daylight saving time, and a map with countries that do or do not use the daylight saving standard.

A list of large cities around the world and their current time

An interactive world map with current times