lat/lon to time offset database

[Larry Ober]

Hi Guy,

Oh I'm not sure. It may have been Microsoft. I was too naive when I found 
it. I thought that I had the answer.

There are about 579 pairs of 115 EST/EDT "pairs" I was hoping this was the 
zone description of some sort.

I was pretty convinced that the 115 part referred to the US eastern time 
zone. The file seem pretty logical to follow starting with a 2 character 
country code on down. All US eastern time zone entries contain 115 in one of 
the columns

Thanks for pointing out that the offset is in seconds!

Regards

Larry

----- Original Message ----- 
From: "Guy Harris" <guy at alum.mit.edu>
To: <tz at lecserver.nci.nih.gov>
Sent: Thursday, February 03, 2011 2:31 PM
Subject: Re: lat/lon to time offset database


>
> On Feb 3, 2011, at 10:33 AM, Larry Ober wrote:
>
>> I don't really need the geo political information like country or city.
>> However it seems like tz is structured continent/country/city/...
>
> To quote the tz-link.htm file in the tzcode package:
>
> Each location in the database represents a national region where all 
> clocks keeping local time have agreed since 1970. Locations are identified 
> by continent or ocean and then by the name of the location, which is 
> typically the largest city within the region. For example, 
> America/New_York represents most of the US eastern time zone; 
> America/Phoenix represents most of Arizona, which uses mountain time 
> without daylight saving time (DST); America/Detroit represents most of 
> Michigan, which uses eastern time but with different DST rules in 1975; 
> and other entries represent smaller regions like Starke County, Indiana, 
> which switched from central to eastern time in 1991 and switched back in 
> 2006.
>
>> Now maybe I already have what I need but I just don't understand it. I 
>> have a
>> copy of timezone.csv that seems to include data from country code to city 
>> etc.
>
> If I search for timezone.csv on Google, I get hits for at least two 
> different data sets:
>
> http://code.google.com/p/x-wrt/source/browse/trunk/package/webif/files/usr/lib/webif/timezones.csv?r=2936
>
> which doesn't seem to include any coordinate information, and
>
> http://msdn.microsoft.com/en-us/library/aa458853.aspx
>
> which says the timezone.csv file "contains the Timezone Index, GMT 
> Differential, and Description for each time zone".
>
> Where did you get that timezone.csv from?
>
>> This seems to point to what looks like coordinate data that I don't 
>> understand.
>>
>> An example for America/New York seems to point to an area 115. There then 
>> seem
>> to be multiple 115 entries like the following;
>>
>> 115, 3907893600, -18000, EST, 0
>> 115, 3919388400, -14400, EDT, 1
>>
>> My interpretation is that 115 is the area, the next column should be some 
>> sort
>> of latitude, next seems to be called an offset in other contexts, next is 
>> the
>> used name of the zone followed by a true/false bit.
>
> *My* guess is that 115 is some representation of an area, 3907893600 is 
> some representation of a time when the US Eastern time zone switches to 
> Standard time, -18000 is the offset in seconds from UTC to use for the 
> time zone after that time, "EST" is the abbreviation to use for the time 
> zone after that time, and 0 is a "Daylight Savings Time is not in effect" 
> indication.
>
> I.e., if you sort all the entries for a given area by the second field, 
> it's a list of standard <-> {daylight savings, summer} time transitions, 
> with each entry saying when the transition occurs, what the offset from 
> GMT in seconds is after that transition, what the zone abbreviation is 
> after that transition, and whether you're in standard or {summer, daylight 
> savings} time after that transition.
>