What is the difference between and 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

Answer 1

pubDate:

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

lastBuildDate:

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

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