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Another good question.  Lets follow it up with another aspect.  Why is Europe 
changing to DST one week before the US and back to standard time one week 
after the US?  Why is Turkey's local time is 30 minutes different from the 
surrounding countries.  Japan and Korea do not have DST at all (but because 
of the physical size of the countries, they don't really need it).

I know that Europeans like to do things differently than we do here, but this 
causes additional confusion when your business spans multiple time zones.  I 
support computers in 22 time zones at some 290 locations.  I really have to 
pay attention to local times when I am calling (or being called by) someone 
to discuss issues/problems and resolutions.

Many folks make reference to GMT and UTC as if they are the same, but they 
aren't quite the same.  GMT is the time as established by the Royal Naval 
Observatory in Greenwich, England.  GMT is sometimes termed Zulu or Zulu time 
(no longer in common use but I still read it in US Coast Guard and Navy 
manuals).  If you have a problem remembering how the conversion goes between 
GMT/ZT and your local time zone (and you know your offset from Greenwich) 
look at the table located at: 
http://wwp.greenwichmeantime.com/info/time-zones.htm - this is a simple 
lookup that helps.

It does not take into account the differences between DST and Standard Zone 
Time - for that look at http://wwp.greenwichmeantime.com/info/timezone.htm

Now, to the question, GMT/ZT, UTC, IAT or UT1?  GMT and Zulu - no issues just 
different names.  UTC (Coordinated Universal Time) is based on IAT 
(International Atomic Time) and only differs by some integer number of 
seconds.  Then we come to UT1 - the NAVIGATORS TIME.  This is not based on 
the same set of atomic clocks as IAT or UTC, but is based on the rotational 
rate of the Earth - with all of it's irregularity and slowing down.  Per 
international agreement, there can be no more than 0.9 seconds difference 
between UTC and UT1 - sorry - my watch does not measure that closely - I am 
lucky to remain in the right day!

Currently the difference between UTC and IAT is -32 seconds (I can measure 
this but I am really pushing myself to do so!)

But all of these differences are introduced by the various definitions of a 
"second".  Some important dates (no need to remember them!) in this evolution 
are:
1884 - International Meridian Conference based on a mean solar day at 
Greenwich, England - this established the common Prime Meridian in Greenwich.
1956 - Ephemeris Time: Based on lunar observations
1958 - Atomic Time: Based on clock data from numerous countries - not all the same elements were used
1967 - Atomic Time: based on radiation patterns of the caesium element.
1972 - Co-ordinated Universal Time (UTC) introduced.
1984 - Dynamical time - based on motion of celestial bodies
1986 - UTC replaced GMT.

Now here is the balance of this interesting tidbit of useless history.  Based 
on solar time, there are only four days each year that have exactly 24 hours. 
 These occur on or about December 25, April 15th (just knew that there had to 
be something positive about the US tax day!), June 14th and August 31st.  The 
balance of the days are either longer or shorter.  If you look at the 
differences between solar time and standard zone time, noon may arrive as 
much as 16.3 minutes early or 14.4666 minutes late.  Since earth time is the 
ultimate time, the time police add or subtract leap seconds from UTC to bring 
the IAT back in line with reality.

Need further confusion?  I can add it - after all I am from the Government and I am here to help.

derrick

   

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