[tz] Dealing with Pre-1970 Data

[Guy Harris]

On Aug 31, 2013, at 8:29 AM, Lester Caine <lester at lsces.co.uk> wrote:

> David Patte ₯ wrote:
>> LMT at any longitude is a simple offset calculation from GMT. LMT = GMT +
>> Longitude (as HMS)
> 
> Yes the base offset is a simple calculation. I got a bit carried away with Guy's assertion that there couldn't be one ;)

What I said was that there couldn't be a simple calculation for the offset of *standard time* from UTC:

> 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).


In your message, you didn't mention LMT until *after* you spoke of 1970 (which is a year chosen as a marker for the point in time for which when the database maintainers say "after this point, we'll put more significant effort into trying to keep the data accurate", for reasons given by Paul) and speaking of "this particular change", which was a change that did more than just address the LMT issue.

It might've been less confusing had you said "I do like *your LMT-related changes*", as that would have made it clearer that you were, in the second paragraph of your message, referring to LMT.  (Your *first* paragraph wasn't discussing the LMT-related parts of Paul's change, as 1970 isn't a cutoff date of any sort for LMT.)

So, yes, I was aware that LMT had a simple formula; given that, I'd personally have preferred that it not be in the database, with dates prior to the "start time" in the database handled by whatever means the particular implementor chooses, whether it's "extrapolate the earliest offset-from-standard-time infinitely backwards into the past" or "fail" or, if the particular function takes a longitude and latitude rather than a tzid as an argument, "calculate LMT".