GMT Timezones

GMT Timezones

[toomai]

Hi All,

when it comes to timezones, RedHat distributions clock setting tools refer -
among many others - to GMT timezones. But here the GMT time difference
concept seems reverted. I mean, why a central europe area whose time is
Greenwich Meridian Time 'plus' 1 hour ( i.e., 10AM + 1h = 11AM ) has to be
set to GMT-1 rather than GMT+1 ? Is this a standard behaviour or a RedHat
peculiarity ?

Though my Linux boxes live happily with GMT-1, I would appreciate any help
on this topic.

Thanks.


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Re: GMT Timezones

[- Luis -]

what version of RH do you have?
i have 7.3 and it have a list of countries for set the time zone.

On Tuesday 11 March 2003 14:49, toomai@tiscali.it wrote:
> Hi All,
>
> when it comes to timezones, RedHat distributions clock setting tools refer
> - among many others - to GMT timezones. But here the GMT time difference
> concept seems reverted. I mean, why a central europe area whose time is
> Greenwich Meridian Time 'plus' 1 hour ( i.e., 10AM + 1h = 11AM ) has to be
> set to GMT-1 rather than GMT+1 ? Is this a standard behaviour or a RedHat
> peculiarity ?
>
> Though my Linux boxes live happily with GMT-1, I would appreciate any help
> on this topic.
>
> Thanks.

-- 
Luis Valencia
------------------------
With a PC, I always felt limited
by the software available.
On Unix, I am limited only by my knowledge.
--Peter J. Schoenster

Re: GMT Timezones

[Amod Phadke]

I think you are looking at the GMT concept wrongly. If we assume GMT as '0' (zero). Then if the sun dawns on central europe 1 hr after it dawns on Greenwitch it is considered as - (minus). If sun dawns on a region before it dawns on Greenwitch it is considered + (plus)

eg. Sun dawns on India 5:30 hrs before it dawans on Greenwitch so Indian time in GMT format will be GMT + 5:30

Cheers,
AMOD.
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Re: GMT Timezones

[Glynn Clements]

toomai@tiscali.it wrote:

> when it comes to timezones, RedHat distributions clock setting tools refer -
> among many others - to GMT timezones. But here the GMT time difference
> concept seems reverted. I mean, why a central europe area whose time is
> Greenwich Meridian Time 'plus' 1 hour ( i.e., 10AM + 1h = 11AM ) has to be
> set to GMT-1 rather than GMT+1 ? Is this a standard behaviour or a RedHat
> peculiarity ?

It's standard behaviour. The source file (timezone/etcetera) contains
the following comment:

# We use POSIX-style signs in the Zone names and the output abbreviations,
# even though this is the opposite of what many people expect.
# POSIX has positive signs west of Greenwich, but many people expect
# positive signs east of Greenwich.  For example, TZ='Etc/GMT+4' uses
# the abbreviation "GMT+4" and corresponds to 4 hours behind UTC
# (i.e. west of Greenwich) even though many people would expect it to
# mean 4 hours ahead of UTC (i.e. east of Greenwich).

-- 
Glynn Clements <glynn.clements@virgin.net>

Re: GMT Timezones

[Glynn Clements]

toomai@tiscali.it wrote:

> Though my Linux boxes live happily with GMT-1, I would appreciate any help
> on this topic.

Note that "GMT-1" will be 1 hour ahead of GMT all year round.

If you want to follow daylight saving time rules, you have to use the
correct regional timezone; for Italy, this is probably "Europe/Rome".

-- 
Glynn Clements <glynn.clements@virgin.net>

Re: GMT Timezones

[Ralph Gesler]

Amod Phadke wrote:
> 
> I think you are looking at the GMT concept wrongly. If we assume GMT as '0' (zero). Then if the sun dawns on central europe 1 hr after it dawns on Greenwitch it is considered as - (minus). If sun dawns on a region before it dawns on Greenwitch it is considered + (plus)
> 
> eg. Sun dawns on India 5:30 hrs before it dawans on Greenwitch so Indian time in GMT format will be GMT + 5:30

This is still not a correct explanation. World time zones are numbered
east and west from the Greenwich meridian (0 degrees longitude). Time
zones east of Greenwich are numbered -1 through -12; time zones west of
Greenwich are numbered +1 through 12. The 0th time zone is Greenwich
time. 

Time zones are 15 degrees of longitude wide, with the center of the
zones at 15 degree increments around the globe. That is, for example,
the Central European time zone (-1) extends from 7 1/2 degrees  to 22
1/2 degrees east centered about the 15 degree east longitude. One
exception to the 15 degree division are the +12/-12 zones which are only
7 1/2 degrees each centered about the International Date line (E180 or
W180).

From the above explanation, it should be seen that the time zone
numbering allows easy conversion from local time to Greenwich time by
adding the zone designation to local time: e.g. 0800 in Rome (-1 zone)
is 0700 in London (0 zone) 0800 +(-1) = 0700. Therefore, when you set
your displayed clock to your local time and provide the zone
designation, the kernel can correctly set the time keeping function to
the correct UTC. 

Notice I used UTC _not_ GMT. GMT has not been in general use for a
number of years since the various international standards organizations
established the atomic clock as the standard time reference. The two are
often considered synonymous, but they are not which is the reason for
leap seconds applied irregularly and which can be plus or minus. 

Hope this helps.

Ralph Gesler

Re: GMT Timezones

[toomai]

Explication about POSIX-style timezone internals where resolutive.
I thank everybody.


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