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.