[tz] What data should TZDB offer?

Paul Eggert eggert at cs.ucla.edu
Mon Jun 7 17:22:51 UTC 2021

On 6/7/21 7:57 AM, David Patte via tz wrote:
> In my own code I use 1800 as the transition date between LST and LMT 
> when it's not specified elsewhere.

1800 is surely a bit early, for all but the most 
technologically-advanced locations. In most of the world, solar time 
maintained its supremacy over local mean time well after 1800.

The "when it's not specified elsewhere" intrigues me, though. What sort 
of specification do you have elsewhere?

One can imagine a tzdb extension containing when local mean time came 
into effect at each location. Unfortunately if we added something along 
these lines to tzdb, I expect we'd have to invent nearly every data 
item. It'd be like a good chunk of the pre-1970 data we already have, 
only worse.

Part of the problem is that people in the early 19th century didn't much 
care whether they were using local solar time or local mean time, and 
many towns actually observed a mean-time approximation to solar time. 
Here's a quote from page 15 of Francis Abbott's book "A Treatise on the 
Management of Public Clocks", 3rd ed. (1839):

"Nothing is more common than to place the management and regulating of 
church clocks in the hands of the sexton, without keeping any check upon 
him or allowing him a salary to stimulate him in this important duty; 
the consequence is, that the village clocks throughout the country are 
kept by chance, and generally speaking vary from one quarter to three 
quarters of an hour from mean time."

Abbott also wrote (page 16) that when a sexton periodically checked and 
set a church clock's time, "the usual mode of ascertaining time is by 
the sundial", i.e., the clock was considered to be a good-enough 
approximation to solar time rather to local mean time.

This state of affairs didn't change until the telegraph made it feasible 
for timestamps to be communicated more accurately, and railroads needed 
more-accurate time.

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