[tz] What data should TZDB offer?

David Patte dpatte at relativedata.com
Mon Jun 7 14:57:48 UTC 2021

In the 1600s, pretty much everyone used Local solar time / apparent time 
/ sundial time, except astronomers.  Use of Local Mean time only really 
caught on for the public in the early 1800s. Capturing the date of 
application of LMT for the primary city of a zone is very useful, but 
determining the exact date it occurred is likely quite difficult. 
History is always murky.

In my own code I use 1800 as the transition date between LST and LMT 
when its not specified elsewhere.

On 2021-06-07 10:38, Stephen Colebourne via tz wrote:
> On Mon, 7 Jun 2021 at 10:28, Guy Harris via tz <tz at iana.org> wrote:
>> Note that theory.html says "don't do that":
>>          In short, many, perhaps most, of the tz database's pre-1970 and future timestamps are either wrong or misleading. Any attempt to pass the tz database off as the definition of time should be unacceptable to anybody who cares about the facts. In particular, the tz database's LMT offsets should not be considered meaningful, and should not prompt creation of timezones merely because two locations differ in LMT or transitioned to standard time at different dates.
> If you ask a Java library "what was the offset on 1st Jan 1600 at
> 12:36 pm" they will all provide an answer. My understanding is that
> the same is true of libraries in many other environments. Most systems
> will provide LMT as the answer. Some (eg. the older Java API) will
> provide a non-LMT offset. Those of us on this list know that the
> question being asked is pretty stupid, but it is asked by end users.
> And when it is asked, people are surprised by the LMT answer. I is
> worthwhile considering if LMT is the best solution to the far past
> problem.
> The second point is not relevant - the proposal is to determine which
> zones get a LMT and history based on post-1970 data with an ISO
> country minimum. There is no proposal to create timezones solely for
> LMT.
> Stephen

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