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I have searched every for an answer to this question and have had no luck. The RSS 2.0 specs do not help.
Long answer is that looking through the RSS 2.0 specification gives you;
All date-times in RSS conform to the Date and Time Specification of RFC 822, with the exception that the year may be expressed with two characters or four characters (four preferred).
You can look at RFC 822 via the link below (specifically part 5), or just google the document name for further guidance.