When is an article 'old'?

Sorry again. I don't like automatic functions, but for the documentation... (I shall be quite knowledgeable by the time I finish.)

When is an article 'old'? I always thought that when it was no longer in the feed which is on the server, but I am not so sure now.

In other words: if I select 'Automatically delete old articles', but do not constrain it (unread, days), what/which articles will be deleted?

And what happens to articles without date or with invalid days?

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