You must add the timezone - follows validator help file
foo must be an RFC-822 date-time
Invalid date-time. The value specified must meet the Date and Time specifications as defined by RFC822, with the exception that the year should be expressed as four digits.
Change the date-time format to comply with RFC822. Here are examples of valid RFC822 date-times:
<pubDate>Wed, 02 Oct 2002 08:00:00 EST</pubDate>
<pubDate>Wed, 02 Oct 2002 13:00:00 GMT</pubDate>
<pubDate>Wed, 02 Oct 2002 15:00:00 +0200</pubDate>
Common errors include localizing the month name or the days of the week, not using the exact abbreviation provided by the specification, or attempting to use a time zone name that is not listed. See Section 5.1 for the list of allowable values for these portions of the date.
Here is an example of an invalid RFC822 date-time. This is commonly seen in RSS 1.0 feeds generated by older versions of Movable Type: