As documented in the weather API <http://developer.yahoo.com/weather/>, the lastBuildDate, pubDate, and condition:date are supposed to use a RFC822 compatible time format. The actual reported time in the weather RSS feeds however do not adhere to the spec.
Example: The RSS feed reports a lastBuildDate of "Fri, 28 Aug 2009 4:53 pm CDT" which is invalid. It should instead use the 24-hour military time format e.g. "Fri, 28 Aug 2009 16:53 CDT", as defined in the spec.