does Calendar understand DST rules for past years?

[Richard Stallman]

I asked Glenn Morris to look at this, but since he is away for a couple
of weeks, I ask if anyone else can figure out what is right here.

man/calendar.el says

      The dates used by Emacs for holidays are based on @emph{current
    practice}, not historical fact.  Historically, for instance, the start
    of daylight savings time and even its existence have varied from year to
    year, but present United States law mandates that daylight savings time
    begins on the first Sunday in April.  When the daylight savings rules
    are set up for the United States, Emacs always uses the present
    definition, even though it is wrong for some prior years.

but the code seems to test calendar-dst-check-each-year-flag, which is
normally true.  I do not understand the code which that calls; it
needs a lot more comments.  Can you please add comments?  But I can't
see any sense in checking each year unless there is a facility for
recording the changes in rules between years.  In that case, the drawback
which is documented is not reallky true.