Last night at dinner a tangential conversation got us on the topic of GMT vs. UTC. None of us knew the exact difference between the two except for that UTC was newer and more exact. So, I looked it up. Here is a good, brief description from the State University of New York Institute of Technology's Science &Amp; Tecnology Society (man that is a mouthful). Basicly it comes down to GMT being based on the rotation of the earth around its axis and the sun (which isn't completely regular) and UTC being based on a Cesium atomic clock (which is far more accurate and regular). UTC is regularly modified with "leap seconds" so that it matches up to GMT to be the standard for date/time stamps. Also, UTC is the authoriative measurement for calculations involving duration.
Update: Corrected, which unit gets leap seconds added. Thanks Daniel for pointing out my mistake.
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Comments (5)
Actually, it seems (according to that article) that it is _UTC_ that is modified with leap seconds to keep in sync with the rotation of the earth (and so the apparent time). I'm not an expert, but you should read http://www.apparent-wind.com/gmt-explained.html for more precise info.
Trying to sum-up: Atomic Time is the "objective" time that should be used for calculations, UTC the one that flows regularly like AT, except for the leap seconds. GMT is actually an ambiguous term, but in the most common usage it is simply a synonym for UTC (UTC should be prefered because it is unambiguous).
# Posted by Daniel Bonniot | July 24, 2004 2:19 PM
This is great. UTC is so much more precise it has to be amended regularly to be useful! lol
The real reason of course is that the US military didn't like a time standard named after a foreign town ;)
# Posted by John | July 27, 2006 6:29 AM
what is the difference between GMT and UTC
# Posted by stella rani | October 27, 2006 4:49 AM
"The real reason of course is that the US military didn't like a time standard named after a foreign town ;)"
Well said Daniel!
# Posted by Mark | April 4, 2008 7:12 AM
Reference:
1) Select "(GMT) Greenwich Mean Time: Dublin, Edinburgh, Lisbon, London" from the Time Zone tab in the Date and Time properties in Windows XP
2) Select "(GMT) Casablanca, Monrovia, Reykjavik" from the Time Zone tab in the Date and Time properties in Windows XP
Question:
Why is #1 above UTC + 1 hour; I thought "Greenwich Mean Time" was synonymous with UTC? Is this a Microsoft issue or is the "(GMT) Greenwich Mean Time: Dublin, Edinburgh, Lisbon, London" currently set to BST (British Summer Time)? As a side note: "(GMT) Casablanca, Monrovia, Reykjavik" appears to be consistent with UTC...
Cheers,
# Posted by JJC | July 15, 2008 11:59 AM