Why not?  That wasn't an "odd time" in Sitka; it was 15:30.  The modern notions of UTC and, moreover, standardized time zones would not have been a contributing factor at the time.  And while it may not have been an exact time spelled out in the treaty (and, indeed, it does not appear such a time was even specified), it certainly represents a distinct transfer of power, which could reasonably be said to be when the treaty actually came into "effect".

Thanks for your work on this, Paul.  I'd put a little effort into a similar patch after the discussion on Asia/Manila in March, but life got in the way of its completion.  I suspect you were piqued by the same sources as I was on this matter.  :)