The weather plugin uses a hardcoded "%y-%m-%d" format currently. I think it is appropriate to use the locale's date representation. For example in German this would be $ date +%x 13.03.2013 instead of 13-03-13.
The plugin uses ISO8601 (with the "T" delimiter omitted) for consistency reasons and because I found it prettier and clearer than the localized strings.
All rifgt, thanks for the fast answer.