How do you generate POSIX time zone strings?

[Paul Eggert]

   Date: Wed, 19 Mar 1997 13:19:11 -0800 (PST)
   From: Tony Glenning <tony at hape.Eng.Sun.COM>

   For example, if this function were run on machine which has TZ set
   to US/Pacific in 1997 it would generate:

	   PST8PDT7,97/3:00:00,300/1:00:00		[*]

First, perhaps you could explain a little why you want to do this,
since you lose information when you generate Posix strings from
Olson-style strings.

Second, assuming you want to do this sort of thing, why would you want
to generate [*]?  It's better to generate the following instead:

	PST8PDT7,M4.1.0/2:00:00,M10.5.0/2:00:00		[**]

since [**] will work in years other than the current year.  [**] isn't
as good as US/Pacific, but it's better than [*].

Here's how to generate [**] without parsing the zoneinfo files.  Run
GNU Emacs, load-library cal-dst, and invoke (calendar-current-time-zone).
If TZ is US/Pacific, it should return

	(-480 60 "PST" "PDT"
	 (calendar-nth-named-day 1 0 4 year)
	 (calendar-nth-named-day -1 0 10 year)
	 120 120)

and it's easy to generate [**] from this value.

If you can't use GNU Emacs in your application, look at cal-dst's
source code for ideas about how to do it.  GNU Emacs doesn't parse the
zoneinfo files; it just uses ANSI C localtime and gmtime like a good
citizen.