Michael H Deckers wrote:
• an odd time (1867-10-19T00:31:13Z) of a local event in Sitka is certainly not the effective time of an international treaty like the Alaska purchase;
We have a letter documenting 15:30 local time, written by General Lovell Rousseau, the American commissioner in charge of the formal transfer of control. He uses the word "precisely" for the ceremony's starting time, presumably to document the point of transition. The treaty itself (and I've read its English version) does not specify a transition date or time, presumably because the Russians wanted to be sure they'd be paid before ceding control.
• assumptions that the day of the week was set back from Saturday to Friday at 15:30 or at 15:33:32 local time
I don't think that's how things happened. Rousseau documents his expedition purely from an American timekeeping point of view, and I have little doubt that the Americans in Sitka kept American time throughout. So in practice, Sitka had overlapping time zones (much like Ürümqi today) for at least a day or two. We don't want to create a new time zone entry just for this problem: we need a single point of transition. The best single point I can think of is the 15:30 time of the Ossipee's salute.
• the assumption that various remote places in Alaska in 1867, not connected by telegraphy lines
You're right, of course that's ridiculous. Of the tzdb Alaska locations only Sitka was inhabited, so none of the other locations even matter from our point of view. If we knew when the other locations became inhabited, I suppose we could use the -00 convention for them at the proper point. However, if we did that, we'd lose relevant information in the database that presumably does reflect a common attitude of passers-through before and after 1867, and this is why I kept the (mostly notional) transitions for the other Alaska locations.