[tz] What is LMTZ?

Paul Eggert eggert at cs.ucla.edu
Wed Sep 18 18:18:10 UTC 2013

On 09/18/13 09:17, Lester Caine wrote:

> Hopefully we can agree that pre standardisation, people based their
> local time on 'midday' which is what what Local Mean Time is.

No, traditionally 'midday' was when the sun was highest, which
corresponds to solar time.  Local Mean Time is a modification of
solar time -- it is solar time averaged out over the year, with
equal-length 24-hour days.

Many locations changed from solar time to mean time at some point
during their histories, but no attempt is made to record these
transitions in the tz database.

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

Yes, that happened in some regions (France and Switzerland
are two examples, the Swiss with two mean-time time zones)
but not all.  For example, it did not happen in California,
as far as I know.

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

Yes, it was first implemented in the 20th century (invented in the 19th).

> So prior to that all time was 'mean time'

No, a lot of it (if you go back in history) was solar time,
which depends on latitude as well as longitude.  Of course
most people way back when didn't care about the difference between
solar time and mean time; but there is a difference.

> 1915 started messing things up with the first varying time rules?

Yes (though it was actually 1916, not 1915).

For more about solar time versus mean time, please see Wikipedia:



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