How accurate is the update-data for the bookmarks?

This depends on the data provided by your blog and your bookmarked sites. Imagine this: a new article was written on June, 15th 2008 at 09:00 hrs PDT (realtime), but the blog’s feed provides the timestamp of June 15th 2008 at 08:00 hrs CET. The plugin will use 08:00 hrs CET to calculate the age of the last update.

And it is the same with the date provided on your blog’s server. If this date is inaccurate, the information provided by this plugin will be inaccurate.