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Re: does Calendar understand DST rules for past years?

From: Glenn Morris
Subject: Re: does Calendar understand DST rules for past years?
Date: Sun, 24 Dec 2006 19:07:12 -0500
User-agent: Gnus (www.gnus.org), GNU Emacs (www.gnu.org/software/emacs/)

Richard Stallman wrote:

> 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.

I can at least reply to mail, even if I can't access CVS.

> 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. 

The above manual text is out of date following my recent changes to
cal-dst (one month ago?). Above is how it used to work. Now if the
flag is true (as you say), it checks each year as needed whenever the
dst start/end dates for the year are first required. Then it caches
the results for that year and reuses them if needed.

If the flag is false, it works as the manual says.

> I do not understand the code which that calls; it needs a lot more
> comments. Can you please add comments?

Comments not critical, can add those on my return.

> But I can't see any sense in checking each year unless there is a
> facility for recording the changes in rules between years. 

Results for each year stored in the cache variable I added a month or
so ago.

> In that case, the drawback which is documented is not reallky true.

Not sure what drawback is. Above manual text is now incorrect when
flag is true. Should have updated the manual when I made my changes,

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