What is the difference between UTC and GMT???

Greenwich Mean Time
-------------------

o Greenwich Mean Time is a time scale based on the apparent motion of
the "mean" sun with respect to the meridian through the Old Greenwich
Observatory (zero degrees longitude). The "mean" sun is used because
time based on the actual or true apparent motion of the sun doesn't
"tick" at a constant rate. The earth's orbit is slightly eccentric
and the plane of the earth's orbit is inclined with respect to the
equator (about 23-1/2 degrees) hence at different times of the year
the sun appears to move faster or slower in the sky. That's why an
uncorrected sundial can be "wrong" (if it is supposed to be telling
mean time) by up to 16 minutes. So if the mean (i.e. corrected) sun
is directly over the meridian through Greenwich, it is exactly 12 noon
GMT or 12:00 GMT (Prior to 1925, astronomers reckoned mean solar time
from noon so that when the mean sun was on the meridian, it was
actually 00:00 GMT. This practice arose so that astronomers wouldn't
have a change in date during a night's observing. Some in the
astronomical community still use the pre-1925 definition of GMT in the
analysis of old data although it is recommended that the term
Greenwich Mean Astronomical Time now be used to refer to time reckoned
from noon.)

o Mean time on selected meridians 15 degrees apart is generally known as
standard time. For example, Eastern Standard Time (EST) is the mean
solar time of the meridian at 75 degrees W.


Universal Time
--------------

o In 1928, the International Astronomical Union recommended that the
time used in the compilation of astronomical almanacs, essentially
GMT, or what was also sometimes called Greenwich Civil Time, be
referred to as Universal Time (UT). The terms "Universal Time" and
"Universal Day" were introduced at the various conferences in the
1800's held to set up the standard time system.

o There are actually a couple of variants of UT. UT as determined by
actual astronomical observations at a particular observatory is known
as UT0 ("UT-zero"). It is affected by the motion of the earth's
rotation pole with respect to the crust of the earth. If UT0 is
corrected for this effect, we get UT1 which is a measure of the true
angular orientation of the earth in space. However, because the earth
does not spin at exactly a constant rate, UT1 is not a uniform time
scale. The variation in UT1 is dominated by seasonal oscillations due
primarily to the exchange of angular momentum between the atmosphere
and the solid earth and seasonal tides. In an effort to derive a more
uniform time scale, scientists established UT2. UT2 is obtained from
UT1 by applying an adopted formula that approximates the seasonal
oscillations in the earth's rotation. However, due to other
variations including those associated with the secular effects of
tidal friction (the earth's spin is continually but gradually slowing
down), high frequency tides and winds, and the exchange of angular
momentum between the earth's core and its shell, UT2 is also not a
uniform time scale.

o So rather than base our civil time keeping on the rotation of the
earth we now use Atomic Time, time based on the extremely constant
frequency of a radio emission from cesium atoms when they change
between two particular energy states. The unit of Atomic Time is the
atomic second. 86,400 atomic seconds define the length of a nominal
"reference" day -- the length of the day as given by the earth's
rotation around the year 1900. But because of the variations in the
earth's spin the length of the actual day can be shorter or longer
than the nominal day of 86,400 seconds. The time scale based on the
atomic second but corrected every now and again to keep it in
approximate sync with the earth's rotation is known as UTC or
Coordinated Universal Time. The corrections show up as the leap
seconds put into UTC from time to time - usually on New Year's Eve.
With these leap second adjustments, UTC is kept within 0.9 seconds of
UT1. Currently, the need for leap seconds is primarily due to the
effects of tidal friction. The earth's rotation in space is monitored
by the International Earth Rotation Service (IERS) in Paris, France,
using a global network of satellite and lunar laser ranging, very long
baseline interferometry, and Navstar Global Positioning System (GPS)
stations. The IERS, in consultation with the Bureau International des
Poids et Mesures in Sèvres, France, determine when a leap second is
needed.

o In 1928, when the term Universal Time was introduced, variations in
the earth's spin were not yet known. So the term GMT was, in essence,
replaced by UT1. Despite the official adoption of the term UT, the
navigational publications of English-speaking countries retained the
term GMT as a synonym for UT1 for some time. So, even today, in
astronavigation, GMT can imply UT1. But in general usage (including
that of shortwave broadcasters such as the BBC, for example), GMT now
usually means the civil (atomic-second-based) time kept in the United
Kingdom which is the standard time of the time zone centred on the 0
degree meridian. In this (the most common) usage, the terms GMT and
UTC are identical. But because there are two possible meanings for
GMT differing by up to 0.9 seconds, the term GMT should not be used
for precise purposes -- particularly not in reference to GPS
observations!

The Origin of UTC
-----------------
o The concept of a coordinated universal time was introduced in 1960
when the British and American national time services initiated a
program to coordinate the offsets of the frequencies and epochs
(phases) of transmitted time service radio signals from Atomic Time in
approximating UT2. Subsequently, other national time services joined
the program. The BIH was charged with the task of monitoring and
maintaining the program and introduced the term Temps Universel
Coordinné or Coordinated Universal Time for the coordinated time scale
in 1964. Initially, the time scale was derived by offsetting its rate
from that of Atomic Time to agree with the average rate of UT2 over
the past year and was held fixed at that rate for the following year.
If the rate of UT2 changed significantly during the year, then an
offset (from 1962, in multiples of 100 milliseconds) could be
introduced on the first day of a month. This system of frequency and
epoch offsets was continued until 1972 when the current practice was
adopted of keeping the rate of UTC equal to that of Atomic Time and
introducing leap seconds when needed to keep UTC to within 0.9 seconds
(it was 0.7 seconds until January 1975) of UT1.

o Sometimes the term "World Time" is used to denote UTC. This strange
and potentially confusing term ("UTC for dummies"?) should be avoided.
Similarly, there is no clear need for the Swatch watch company's
recently introduced "Internet Time" (Central European Time measured in
1/1000 of a day (a "beat")).

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. So, check your GMT Linux box and change it to UTC.










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.

http://www.diffen.com/difference/GMT_vs_UTC

Gentlemen: Thanks for all the erudite information about UTC and GMT. I actually knew that stuff.
I wasn't really looking for this level of definition of the concepts of UTC and GMT, I just wondered why two Linux installs which were both set up exactly the same way, through the Linux Date and Time Admin menus. I have looked at the two Linux boxes and cannot see any difference. Why is one labelling its time UTC and the other GMT?
Thanks - Frank

Erase any clockmode line if any, this is deprecated. Stick with the UTC=TRUE or YES.


http://www.redhat.com/docs/manuals/enterprise/RHEL-4-Manual/en-US/Reference_Guide/s2-sysconfig-clock.html

Thanks, enrique and Tintin.

It seems we may be on the right track here. No solution yet, but...

I looked at the /etc/sysconfig/clock file in two Linux boxes,

In the "good" system:

# The ZONE parameter is only evaluated by system-config-date.
# The timezone of the system is defined by the contents of /etc/localtime.
ZONE="Etc/UTC"
UTC=true
ARC=false

This one respnds like this:
#date
Tue Dec 30 16:07:40 UTC 2008


In the "bad" system:

# The ZONE parameter is only evaluated by system-config-date.
# The timezone of the system is defined by the contents of /etc/localtime.
ZONE="Europe/London"
UTC=true
ARC=false

Whereas this one responds like this:
Tue Dec 30 16:08:20 GMT 2008

Neither one has the deprecated configuration "CLOCKMODE=GMT"

So it seems that 'ZONE="Etc/UTC"' is the preferred configuration.

And it appears that the localtime file is a copy of the UTC file on one system and a copy of the GB file in the other case.

So I tried changing the /etc/sysconfig/clock file  on the GMT system to 'ZONE="Etc/UTC"' and I re-sourced rc.local and restarted the ntpd service. This one STILL responds like this:

#date
Tue Dec 30 16:23:16 GMT 2008

So I still don't know how to get it to respond with
Tue Dec 30 16:23:16 UTC 2008
instead.

I even rebooted the system and it still responds with GMT.

Any idea what configuration file REALLY controls this????

Thanks - Frank

Thanks. No one suggestion was the silver bullet but the information was very useful. TinTin went a little overboard actually defining the difference, but that was OK.

The final solution was extremely easy. First, I never noticed before, but down at the bottom of the list of timezones in the Gnome Desktop: System->Admin->Date&Time menu there is a heading called "Non-geographic Time Zones".

Once I selected that, date started showing time as UTC. Previously I selected London Time and clicked on the "Use UTC Time" check box. That apparently isn't good enough. I haven't been able to figure out the difference between these two approaches, both result in to =
/etc/sysconfig/clock looking exactly like this:

ZONE="Etc/UTC"
UTC=true
ARC=false

It would be nice know why I have resort to the desktop to achieve this, seems like the command line and various time realted scripts ought to tell the whole story, but what do I know?!?!

My clock skew problem resulted from my ntp client QUIETLY not working because my router table didn't include a default  line for localdomain. This seemed to block our ability to access the ntp server.  Once we added that to the router table, the ntp service resumed and that system's clock skew problem went away.

Thanks for all your input.