What is the difference between <pubDate> and <lastBuildDate> in RSS?

Question

I have the feeling, in every RSS.xml file, both the pubDate and the lastBuildDate match.

I am sure that this one, is not always true...

So firstly, what is the difference between those two above?

Secondly, the RSS readers, sort the content by Date, based on the pubDate or the lastBuildDate?

Answer 1

lastBuildDate specifies the last date/time the entry was modified. pubDate specifies the actual publication date/time.

The reason you see these as generally the same is because by the time you get the RSS feed, there hasn't been any edit to the article.

I can't find the RSS spec on this unfortunately, but I am pretty positive that's what they are.

Answer 2

pubDate:

The original publication date for the channel or item. (optional)

lastBuildDate:

The most recent time the content of the channel or item was modified. (optional)

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Here are some docs for the optional items in the RSS 2.0 spec.

Answer 3

You can find the RSS 2.0 specification at http://cyber.law.harvard.edu/rss/rss.html