[tz] Before 1970 - proposal to change LMT

Paul Eggert eggert at cs.ucla.edu
Sun Aug 10 23:59:33 UTC 2014

Stephen Colebourne wrote:

> What I find objectionable is to have a named entity (zone or link)
> where the LMT of the named location is replaced by the LMT of some
> other location.

This objection seems to be based on a misunderstanding of what the LMT 
entries were always supposed to mean.  They never meant anything like 
"the time at this location was exactly 7 hours, 52 minutes and 58 
seconds before GMT" (to use America/Los_Angeles as an example). 
Instead, they meant that the time was not closely specified: different 
people at that location would not have cared about minor differences in 
GMT offsets, or (if pushed to be specific and if knowledgable about the 
topic) would even have disagreed about what the GMT offsets should be.

In hindsight the tz database format should had have a specific notation 
for this.  But it doesn't, so we use LMT entries as stand-ins.  Their 
exact GMT offsets do not matter.  It's similar to the zzz notation that 
we use for places while uninhabited.  I suppose one could argue that the 
LMT and zzz notations are both abuses of the format, but we needed 
*some* way to say that the local time was not closely specified or was 
undefined, and that's what we came up with.

> I propose that all LMT values in the database are replaced by an new
> value, representing what could be described as "averaged/smoothed
> regional far past time".

This substitutes one notation for another.  Why change notations now?

> The net effect of this would be to provide a much more regularized
> value for the far past (pre 1850 or later).

This makes it sound like the proposed notation would be more misleading 
than the current one, as it would suggest an even-more-regularized past 
than the current one does.  We shouldn't give users the incorrect 
impression that long-ago timekeeping was tidy.

More information about the tz mailing list