Understanding Zulu Time and Coordinated Universal Time

When you read weather forecasts and maps, you may notice a four-digit number followed by the letter "Z" somewhere at their bottom or top. This alpha-numeric code is called Z time, UTC, or GMT. All three are time standards in the weather community and keep meteorologists—regardless of where in the world they forecast from—using the same 24-hour clock, which helps avoid confusion when tracking weather events between time zones.

Although the three terms are used interchangeably, there are small differences in meaning.

GMT Time: Definition

Greenwich Mean Time (GMT) is the clock time at the Prime Meridian (0º longitude) in Greenwich, England. Here, the word "mean" means "average." It refers to the fact that noon GMT is the moment on average each year when the sun is at its highest point in the sky at the Greenwich meridian. (Because of Earth's uneven speed in its elliptical orbit and it's axial tilt, noon GMT isn't always when the sun crosses the Greenwich meridian.) 

History of GMT. The use of GMT began in 19th century Great Britain when British mariners would use the time at the Greenwich Meridian and the time at their ship's position to determine the ship's longitude. Because the UK was an advanced maritime nation at the time, other mariners adopted the practice and it eventually spread worldwide as a standard time convention independent of location.

The Problem with GMT. For astronomical purposes, the GMT day was said to start at noon and run until noon the following day. This made it easier for astronomers because they could log their observational data (taken overnight) under a single calendar date. But for everyone else, the GMT day started at midnight. When everyone switched to the midnight-based convention in the 1920s and 1930s, this midnight-based time standard was given the new name of Universal Time to avoid any confusion.

Since this change, the term GMT isn't used much anymore, except by those living in the UK and its Commonwealth countries where it's used to describe the local time during the winter months. (It's analogous to our Standard Time here in the United States.)

UTC Time: Definition

Coordinated Universal Time is a modern version of Greenwich Mean Time. As mentioned above, the phrase, which refers to GMT as counted from midnight, was coined in the 1930s. Other than this, one of the biggest differences between GMT and UTC is that UTC does not observe Daylight Saving Time.

Backward Abbreviation. Ever wonder why the acronym for Coordinated Universal Time isn't CUT? Basically, UTC is a compromise between the English (Coordinated Universal Time) and French phrases (Temps Universel Coordonné). the use the same official abbreviation in all languages. 

Another name for UTC Time is "Zulu" or "Z Time."

Zulu Time: Definition

Zulu, or Z Time is UTC Time, only by a different name.

To understand where the "z" comes from, consider the world's time zones. YEach is expressed as a certain number of hours "ahead of UTC" or "behind UTC"? (For example, UTC -5 is Eastern Standard Time.) The letter "z" refers to the Greenwich time zone, which is zero hours (UTC + 0). Since the NATO phonetic alphabet ("Alpha" for A, "Bravo" for B, "Charlie" for C...) word for z is Zulu, we also call it "Zulu Time."