Here's another patch that I think should be applied.  When parsing a
date, it first tries parse-time-string, and only falls back to using
timezone-make-date-arpa-standard if parse-time-string gives an error.
This is about 4 times faster in my tests with good dates, and shaves
0.6s off of opening a summary buffer with 6000 articles.  I've cc'd 
Dave Love since I believe he introduced this.  If he can confirm that
the problematic dates he encountered triggered an error in
parse-time-string, then this patch should be ok.

Dan