This may be a serendipity bug. The RSS feed is always tagged with incorrect time stamps. For example, if a blog post is created on 2010 Feb 08, 23:00:00, the corresponding feed will be tagged with a timestamp of 2010 Feb 08, 07:00:00.
I figured out that this is because I have set the time zone offset to +8 in serendipity. The RSS feed is adding this offset to the timestamp. In actual fact, there is no need for serendipity to add this offset, because the time stamp is in UTC format, any RSS reader will take care of this properly. I will find some time to report this to the author.