What Time Is It?

Revised 2012-07-31

drama-mask And we say…

GMT (UTC)  Local Time 

The Question

What time is it? An easy question, right? The first impulse is to look at the clock on the wall or the wristwatch. The answer is a little more complicated when you cross continents and time zones and deal with winter and summer or standard and daylight savings time.

The Answer

World Clock

So how do we tell time around the world?

Shortwave broadcasters and radio amateurs use a universal time standard known as UTC (Universal Time Coordinated) or GMT (Greenwich Mean Time). UTC is the term currently in favor, but you will find broadcasters and publications using "Coordinated Universal Time", "Universal Time" (UT), "Greenwich Mean Time" (GMT) and "world time." Military personnel use the term "Zulu" time. For our purposes, all terms mean the same thing.

Further, the 24-hour clock is used to avoid confusion with "a.m." and "p.m." Often the term "military time" is used. The time is expressed as 4 digits (hours and minutes) without punctuation, and 12 hours are added to the hours after 1 p.m. For example, 9:24 a.m. is 0924 and 9:24 p.m. is 2124 (12+9=21).

Greenwich meridian UTC is based upon the time on the 0 degree longitude — the prime meridian — passing through Greenwich, England, home of the original Royal Greenwich Observatory. Richard B. Langley of the Department of Geodesy and Geomatics Engineering at the University of New Brunswick, Fredericton, has written a brief titled A Few Facts Concerning GMT, UT, and the RGO. It is recommended reading.

If you know the number of hours difference between your local time and UTC, then you can convert UTC time to your local time.

One way is to tune into the National Institute of Standards and Technology stations WWV (in Colorado) or WWVH (in Hawaii). These time-and-frequency standard stations broadcast on 2500, 5000, 10000, and 15000 kHz; WWV also broadcasts on 20000 kHz. Time, in "Coordinated Universal Time", is announced every minute. Calculate the difference between your local time and the time announced by WWV/WWVH.

There's one more thing to remember when you are converting time to or from UTC. If the difference in hours rolls over 0000 UTC, you must adjust the day as well. For example, in the USA 0100 UTC Sunday is 2000 Saturday Eastern Standard Time.

Supplemental Reading

  1. USNO's Time & Frequency Services
  2. A Walk Through Time