lat/lon to time offset database

Julian Cable julian.cable at
Fri Feb 4 09:39:31 UTC 2011

Looks like you've got this resolved but I thought I'd add my experience.

Timezones are political, not geographical.

They change all the time, they are not static.

I manage the data feeds to our "how and when to listen" web site for the
BBC World Service.

We map from population centre to timezone name using Geonames and from
timezone name to offset using Olson.



-----Original Message-----
From: Larry Ober [mailto:larry.ober at] 
Sent: 03 February 2011 18:33
To: tz at
Subject: lat/lon to time offset database

I'm a nubie at understanding the ins and outs of timezones so please
forgive me for my potentially dumb questions.

I am looking at building a small embedded system (No OS) that will
require determining the local time from the lat/lon of a location. So
basically I need lat/lon to time offset from "GMT".

I don't really need the geo political information like country or city. 
However it seems like tz is structured continent/country/city/... 

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. 
This seems to point to what looks like coordinate data that I don't

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

If this is basically true, I guess that what I don't understand it how
to "parse" the latitude and how to use the longitude offset.

Of course maybe I have this entirely wrong.

I would appreciate any help on this basic issue.


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