[tz] Dealing with Pre-1970 Data

Lester Caine lester at lsces.co.uk
Sun Sep 1 12:23:41 UTC 2013

Zefram wrote:
>> That is the simple bit, Longitude to an offset from UTC
> Perl script attached for your convenience.
My own use is with PHP timestamps :) ... but you have already made assumptions 
in that script which is the point here.
Silly little things like is 7.5 degrees included in the first or second zone of 
a simple grid. Academic, but I've seen too many cases where 'that will not be a 
problem' comes up in software only to find later that one system is working one 
way and another. 'Rounding' is the bane of many of these things and often it's 
that very rounding that leads to the problems. Uncertainty needs to be managed.

>> >However the one I was really thinking about was the 'equation of
>> >time' calculation in a format we can use as a 'standard'.
> For what purpose do you expect to use*apparent*  solar time?  If you
> really want it, look at astronomical sources.  That's where the
> demand exists for formulae for this and many other phenomena that are
> tangentially related to time.
As I said - this only really relates to historic material. The astronomical 
community has methods of working which perhaps we have to live with, but a 
simpler view of things would suffice for historical comparisons. When converting 
documents that specifically use the sun as their reference. One can probably say 
'Sod it xxx will do', but given the fine tooth comb that is now being applied to 
this data, a standard consistent with the other time standards we are using is 
essential. This is why the current statement that the tz database is 'invalid 
prior to 1970' causes such a problem!

Lester Caine - G8HFL
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