While this date-time value is formatted correctly, it either appears to be in the future, or far enough in the past to indicate that there might either be a software error, or an incorrectly set clock, involved.
Instead, many clients sort by dates, either by default, or as an option. For this reason, it is important that date values are as accurate as possible.
One common error, particularly among the British and Western Europe during the summer time (i.e., during the periods of BST and WEST), is for the numeric part of the time to reflect local time and for the zone part of the time to hard code GMT.
Ensure that RSS 2.0 pubDate and atom:updated and atom:published elements are as accurate as possible.
Not clear? Disagree?
You might be able to find help in one of these fine resources.