Coordinated Universal Time (UTC)

10 January 2019

The concept of time is universal among humans, but telling time precisely has not always been important. The movement of the Sun through the sky has been the traditional way to tell time, either by just observing the movement of the Sun or with devices such as sundials and time sticks. Although devices such as hourglasses, candle clocks, and water clocks were also used to tell time more precisely for short periods.

Before people could quickly travel or communicate over long distances, there was not much need for standardized time. The town clock would be the official time in a town, and it would be synchronized with the position of the Sun. When railroads and steamships became common in the 19th century, it became very important to have a commonly agreed-upon time in different places so that their arrivals and departures could be properly scheduled.

In Britain, the country’s clocks were all synchronized to Greenwich Mean Time (GMT), which is the time in Greenwich, England where the Royal Observatory is located. A telegraph or radio at the official clock in GMT would send out a signal every hour, so that other clocks could be set to match the official clock. Following England’s lead, other countries also began setting up standardized local times, creating time zones so that sunrise and sunset happen at approximately the same local time for everybody living in an area. By 1883, Canada and the United States had adopted a standard time system using five time zones, much like the ones we use today.

Standard World Time Zones
Figure 1: Standard time zones of the world. (Image by Hellerick, based on earlier image by CIA with many modifications by other contributors, TimeZonesBoy (Based on File:CIA WorldFactBook Time Zones.svg) [Public domain], via Wikimedia Commons)

Coordinated Universal Time (UTC), also called Zulu Time by the military and aviation fields, is the system that people now use to keep time. UTC combined with time zones that divide the world into twenty-four more or less equal vertical sections along lines of longitude (how far you are east or west of the Prime Meridian, which is at 0 degrees longitude) let people know the time at any place on Earth. Additionally, astronauts in space use UTC to tell time on board their spacecraft. Time zones are designed to allow for sunrise and sunset to occur at approximately the same local time, regardless of where you are in the world at a given latitude (how far north or south of the Equator you are).

Latitude and Longitude
Figure 2: Latitude and Longitude. (Image by Djexplo[Public domain], via Wikimedia Commons)

You can calculate local time anywhere on Earth if you know what the UTC is and what your local time zone offset (difference from UTC) is. For instance, if the UTC is 11:00 and you are in the Eastern Standard Time zone (UTC -5, or five hours behind UTC), then your local time is 06:00 (11 – 5 = 6). You can also use online tools such as the Time Zone Converter on to figure out what your local time is compared to UTC.

Most of the world, including the world’s space organizations, also use what is known as a 24-hour clock to tell time. This is sometimes called “Military Time,” and is used to reduce the chances of mistakes in time if AM or PM are not written or accidentally mixed up. Unlike the 12-hour clock, which starts at 12:00AM (midnight) and goes until 11:59 AM and then starts again at 12:00PM (noon) and goes until 11:59PM, the 24-hour clock starts at 00:00 (midnight) and goes to 23:59 every day. The time is sometimes written without a colon between the hours and minutes, which would look like this: 2359.

To convert from the 12-hour clock to the 24-hour clock, simply add 12 to the hours if the time is in the afternoon or evening (PM). For instance, 3:00PM in the 12-hour clock system would be 15:00 in the 24-hour clock system.

Did you know? Canadian engineer and inventor Sandford Fleming proposed the concepts of worldwide standard time, the 24-hour clock, and world time zones in 1879

Computers also use a 24-hour clock that includes seconds and milliseconds (thousandths of a second). A date-time stamp on computer data might look like this:


The date comes first [2018 (year) - 10 (month - October) - 04 (day)] and then the specific time comes next [T (time) 17 (hour,17 or 5PM): 36 (minutes) : 00.00 (seconds and milliseconds)], followed by a “Z” which stands for Zulu Time or UTC.


Creet, M. (1990). Sandford Fleming and Universal Time. Scientia Canadensis, 14(1-2), 66-89. Retrieved from

Current UTC — Coordinated Universal Time. (2018). Retrieved from

ISO 8601 Date and time format. (2018). Retrieved from


This is content has that been provided for use on the CurioCity website.