How to change mail/thunderbird date format?

[Rhialto <rhialto%falu.nl@localhost>]

I just installed mail/thunderbird from the stable pkgsrc branch.
It shows dates as 08/18/08 which I find quite annoying; I want either
2008-08-18 or 18-08-2008 (preferably the latter).

Googling suggests that one should be able to set LC_TIME=en_GB or some
other country, which thunderbird is supposed to obey. See for instance
http://leuf.net/ww/wikidn?ThunderbirdAndDateFormat or
http://kb.mozillazine.org/Date_display_format#Configuring_the_date.2Ftime_system_settings_on_your_computer
 .

Unfortunately this does not seem to work.

I tried all kinds of variants including the character set as
en_GB.ISO8859-1 or en_GB.ISO88591 or en_GB.iso-8859-1.

Does anyone have some other idea?

-Olaf.
-- 
___ Olaf 'Rhialto' Seibert      -- You author it, and I'll reader it.
\X/ rhialto/at/xs4all.nl        -- Cetero censeo "authored" delendum esse.