Not sure why folks want to change the pubDate format given in a newsfeed. I have nothing but trouble when I try to base anything on pubDate. I can understand the desire to show feeds with localized date/time, but seems like a whole lot of trouble to go through. My experience with most feeds generated is that they don't even reflect correct information in about 80% of the usage I've seen. The discrepency usually can be attributed to developers not understanding how to correctly use date/time fields and convert them, and the further complication of date/times being formatted in databases based on the server date/time setup, which may not be correctly setup or maintained.
It's really puzzling to me how RSS specs decided to use the RFC822 date/time format in the first place. This was a standard developed for first gen email text messages. Peter's correct in that it was an American originated spec, but it did have a group of international folks working on it, which is why the format doesn't even match American date formats either.
Now, you can change the format using XSL, but it's a real chore to provide a solution that's universal and fully globalized. Most solutions are going to be local solutions as far as I can tell. I've done a bit of that stuff, but reverted back to not caring about it because most feeds don't always give you pubDate in the rfc822 format, and trying to figure out universal routines to deal with all the variances of gloablized date/time formats is difficult to say the least. So, I just take it as it is - much like the content in a feed.
I don't even like to validate the pubDate format because I'd rather worry about getting the content than stressing over whether the pubDate is valid and miss displaying the content. Maybe I'm funny that way, but my usage of feeds is not based on pubDate, and if I choose to cache, I think the better approach would be to simulate a ttl element based on the date/time I pulled the feed originally. I can display the pubDate as given, and still have control over timing for feed retrieval and display. Just my thoughts...