From mboxrd@z Thu Jan 1 00:00:00 1970 From: Ralph Gesler Subject: Re: GMT Timezones Date: Tue, 11 Mar 2003 22:46:40 -0800 Sender: linux-admin-owner@vger.kernel.org Message-ID: <3E6ED7D0.E98F806F@pacificnet.net> References: <20030312040409.23338.qmail@operamail.com> Mime-Version: 1.0 Content-Transfer-Encoding: 7bit Return-path: List-Id: Content-Type: text/plain; charset="us-ascii" To: Amod Phadke Cc: toomai@tiscali.it, linux-admin@vger.kernel.org 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