Re: GMT Timezones

[Ralph Gesler <rgesler@pacificnet.net>]

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