[tz] What is LMTZ?

Lester Caine lester at lsces.co.uk
Fri Sep 20 19:39:49 UTC 2013

Paul Eggert wrote:
> Lester Caine wrote:
>> I'd almost argue that NO historic data is better than a SINGLE subset arbitrarily selected?
>> Currently there is no mechanism to tell you that the historic data may not actually apply at all?
> These are both valid points.  It's certainly easy to
> remove data before 1970 automatically, and some platforms
> do that now (QNX, for example), so the first option is
> already there and some platforms already take it.
> As for the second, we could add a comment to each
> zone specifying the earliest time stamp such that
> the zone is known to be valid for all locations
> in its region after that time.  The default would
> be 1970, which means the tz database trivially
> conforms to this pattern now.
> I see two problems with this idea, though.  First,
> it needs to be integrated with winnowing, and it's
> not yet clear how that would work.  Second, and more
> important, except for Europe/London and perhaps
> a handful of other places, we wouldn't know what
> value to put into the comment, other than
> the default "1970".  This seems a fairly serious
> objection.

I think we have reached the point where it's clear that TZ is never going to 
support anything before 1970 with any reliability. winnowing is a pointless 
exercise unless you are prepared to add new data later. So the only SAFE way to 
distribute a sub-set of TZ data is not to link it to past history that it is not 
100% related to. The only problem remaining is how to allow a correct historic 
record to be used with the restricted TZ data.

Lester Caine - G8HFL
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