[tz] What is LMTZ?

Alan Barrett apb at cequrux.com
Wed Sep 18 19:19:24 UTC 2013

On Wed, 18 Sep 2013, Lester Caine wrote:
>Alan Barrett wrote:
>>>> You often mention "LMTZ".  What does that term mean?
> Hopefully we can agree that pre standardisation, people based 
> their local time on 'midday' which is what what Local Mean Time 
> is. and different locations have a different offset. Anything on 
> the same longitude will be the same time.

OK so far, except for the difference between solar time (where 
clocks are synchronised to midday every day, and days are not all 
the same length because the elapsed time between one solar midday 
and the next varies from day to day throughout the year) and mean 
time (where clocks are synchronised to a notional "mean" midday, 
and days are all the same length).

> When people started standardising they agreed on a Local Mean 
> Time that applied across a bigger area.

I'd say that they agreed on a time that applied across a bigger 
area.  That time no longer deserved the name "Local Mean Time" 
because, for most locations in the zone, it was no longer the mean 
time that applied to their location.

> I'm probably wrong but I am fairly sure that 'summer-time' or 
> 'daylight saving is a 20th century invention?

Yes, I think so.

> So prior to that all time was 'mean time' and it just depended 
> on which longitude was being used to set it. 1915 started 
> messing things up with the first varying time rules?
> Up until 1915 all we need to know is how the 'offset' from LMT 
> was set when groups of locations agreed on a standard ... which 
> is my 'LMTZ' data.

They didn't agree on an offset from LMT.  They couldn't, because 
LMT is different for every line of longitude in the area covered 
by the zone.  They agreed on a place whose local mean time would 
be used throughout the zone, or they agreed on an offset between 
their time and the time in some other place.

Anyway, I think I am starting to get the picture.  When you say 
"LMTZ" you mean something like "A standardised time that applies 
across a geographical zone of significant size."  I think it's 
confusing for you to refer to this time using a phrase that 
includes the words "Local Mean Time".

--apb (Alan Barrett)

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