`format-time-string', TZ and environement variables

`format-time-string', TZ and environement variables

[Lars Magne Ingebrigtsen]

I needed to format a time string in the time zone it was originally
written in (after making it more valid).

(setq time (parse-time-string "16 Oct 92 13:48:29 PST"))
=> (29 48 13 16 10 1992 nil (-28800) -28800)

(Hm.  That's slightly buggy.  The DST should field is documented to be
either t or nil, so that `(-28800)' thing looks weird...)

Anyway, the original date string is kinda invalid because of the Y2K
problem, so I thought I'd just parse it and then recreate the string,
which turned out to be difficult and undocumented.  

(apply 'encode-time time)
=> (10975 14509)

(format-time-string "%a, %d %b %Y %T %z" (apply 'encode-time time))
=> "Fri, 16 Oct 1992 22:48:29 +0100"

Er, nope.

(setenv "TZ" (format "GMT%d" (- (/ (or (car (last time)) 0)
				       60 60))))
=> "GMT8"

(format-time-string "%a, %d %b %Y %T %z" (apply 'encode-time time))
=> "Fri, 16 Oct 1992 13:48:29 -0800"

I think.  Or something.

`format-time-string' allows either formatting in the local time zone, or
using UT.  I think either it should be extended to allow formatting in
any time zone, or the "TZ" trick should be documented.  If the latter,
we should probably have a nice macro for the environment variables, too:

(with-environment-variable ("TZ" "GMT8")
  (format-time-string "%a, %d %b %Y %T %z" (apply 'encode-time time)))

Because using `setenv' directly is kinda yucky.

This is not for Emacs 24.1, obviously...

-- 
(domestic pets only, the antidote for overdose, milk.)
  bloggy blog http://lars.ingebrigtsen.no/

Re: `format-time-string', TZ and environement variables

[Andreas Schwab]

Lars Magne Ingebrigtsen <larsi@gnus.org> writes:

> (Hm.  That's slightly buggy.  The DST should field is documented to be
> either t or nil, so that `(-28800)' thing looks weird...)

Fixed.

Andreas.

-- 
Andreas Schwab, schwab@linux-m68k.org
GPG Key fingerprint = 58CA 54C7 6D53 942B 1756  01D3 44D5 214B 8276 4ED5
"And now for something completely different."

Re: `format-time-string', TZ and environement variables

[Ted Zlatanov]

On Fri, 24 Feb 2012 20:24:38 -0800 Lars Magne Ingebrigtsen <larsi@gnus.org> wrote: 

LMI> we should probably have a nice macro for the environment variables, too:

LMI> (with-environment-variable ("TZ" "GMT8")
LMI>   (format-time-string "%a, %d %b %Y %T %z" (apply 'encode-time time)))

LMI> Because using `setenv' directly is kinda yucky.

I agree it's generally useful to have that macro, except it should take
a list:

(with-environment-variables (("TZ" . "GMT8")) ...)

Ted