[tz] Dealing with Pre-1970 Data
lester at lsces.co.uk
Sat Aug 31 13:51:38 UTC 2013
Guy Harris wrote:
> On Aug 31, 2013, at 12:45 AM, Lester Caine <lester at lsces.co.uk> wrote:
>> I do like this particular change but in does only need to be an agreed standard for taking location information and creating a time offset?
> To which particular change are you referring?
Paul's inclusion of LMT offsets in the main database.
>> The 'equation' can then be used with any location just by feeding the coordinates in, so there is no need for a 'database'.
> I assume you're not asserting here that a simple mathematical formula, with latitude and longitude as variables, that results in the offset of "standard time" from UTC for that location exists, given that no such formula can exists (note the word "simple" there - if it were a simple problem, the tz database wouldn't have been created in the first place).
The simple calculation is the difference between the time when the sun is
directly overhead at Greenwich and at the location being looked at. An
approximation only requires the longitude, but the local mean time needs a
correction based on the date, or rather the time of year.
http://en.wikipedia.org/wiki/Local_mean_time is nice and simple, and a nice set
of calculations are on http://en.wikipedia.org/wiki/Equation_of_time
Any equation is an approximation, but as long as we all work from the same one
then we will get the same result.
>> In fact, any existing 'location' in the database we should be able to access TZ offset, and LMT offset as both have a purpose.
> What do you mean by "LMT offset"?
As has been described. The LMT offset is based on longitude and varies smoothly
around the planet, while timezones set a fixed offset for a large slice of the
planet which may also vary based on latitude. Add in daylight saving, and we
have the whole reason that the tz database exists, but LMT time is totally
independent of that. Paul is simply using a 'snapshot' of the LMT values for
zones where there is no alternative data, and the value stored in the database
may not actually match that created by longitude of the describing city, but
rather a longitude related to the whole zone. And I am assuming that he is not
bothering about the annual cycle in calculation.
Lester Caine - G8HFL
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