Ravings of an Intermittent Fool

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).

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 ;)

what is the difference between GMT and UTC

"The real reason of course is that the US military didn't like a time standard named after a foreign town ;)"

Well said Daniel!

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,

Too me its just ignorance on many programmers that so much information on stuff makes no sense. I am not an expert, but I have seen on software and websites, things as stupid as "Portuguese Euro" or "Deutsche Mark" for Germany, since all these European Union states share the same currency: Euro.

I almost gave up understanding all this and just seek the time town in timeanddate.com which seems very usefull for this, adapting to the daylight saving times. That allows me to understand why I have differences that change from 2 to 5 hours to de same city (depending of season between different hemisphere) as in from Lisbon(here) to Sao Paulo (Brazil).

I set the towns I need to keep a track for phonecalls and IM using a personal clock where you can set a bunch of cities simultaneously.

Greetings from Portugal.
Leonel

Greenwich Mean Time (GMT) is a term originally referring to mean solar time at the Royal Observatory, Greenwich where a system was first developed around 1850 for tracking time based on the rotation of the Earth. It is now often used to refer to Coordinated Universal Time (UTC) when this is viewed as a time zone.

Strictly speaking, UTC is not a time zone but an atomic time scale which only approximates GMT in the old sense. It is also used to refer to Universal Time (UT), which is the astronomical concept that directly replaced the original GMT.

In 1970 the Coordinated Universal Time system was devised by an international advisory group of technical experts within the International Telecommunication Union (ITU). Coordinated Universal Time (UTC,--Fr. Temps Universel Coordonné) is International Atomic Time (TAI) with leap seconds added at irregular intervals to compensate for the Earth's slowing rotation. Leap seconds are used to allow UTC to closely track UT1, which is mean solar time at the Royal Observatory, Greenwich.

The ITU felt it was best to designate a single abbreviation for use in all languages in order to minimize confusion. Since unanimous agreement could not be achieved on using either the English word order, CUT (coordinated universal time), or the French word order, TUC (temps universel coordonné), the acronym UTC was chosen as a compromise.

The difference between UTC and UT1 cannot exceed 0.9 s, so if high precision is not required the general term Universal Time (UT) (without a suffix) may be used.

In casual use, Greenwich Mean Time (GMT) is the same as UTC and UT1. Owing to the ambiguity of whether UTC or UT1 is meant, and because timekeeping laws usually refer to UTC, GMT is avoided in careful writing.

"UTC being based on a Cesium atomic clock (which is far more accurate and regular)."

Wow, this is old, but it has found its way to the top in some Google searches.

Can I just insert that I'm glad you worded things the way you did. It urks me when people say that UTC is more accurate than GMT. It is more regular, but accuracy depends on what you are measuring. The atomic clock is very regular but not so acurate at measuring the length of a day. Tyically we want 24 hours to be the length of a day as measured by the rotation of the earth, so that if we schedule a baseball game at 3pm three years from now, that doens't end up being in the middle of the night. :)

Time is an odd thing, since it really does express more than one idea, and modern technology has allowed us to realize how imprecise so many things are.

You cannot just express 3pm on 12/3/2025 as a certain number of ticks of an atomic clock from now, because the earth isn't as "regular" as the atomic clock. :)

Mark:

Microsoft's naming is a little misleading perhaps. The difference is simply that you're selecting a region rather than an explicitly named time zone: there's no generic name for the UK, Ireland and Portugal's time zone even though they all switch to daylight savings time on the same date.

Mauritania and Casablanca on the other hand do not switch to DST and are therefore always GMT. Hence there needs to be two different settings in Windows.

Of course, if you switch Windows to UK time in winter it *will* show GMT.