In this post, I decribed that I wasn't able to have Google Reader export a feed of the last 24 hours of shared items. The feed that is generated by Google reader uses the publish date from the original article as the pubDate of the item in the feed. It would be great if Google embedded the "shared" date in the feed for each item.
In the mean time, I have created a Yahoo Pipe that will do this. Unfortunately, this requires that the previous period's feed needs to be saved somewhere where this script can access. I have set a cron job to fetch the feed once a day. Then the yahoo pipe uses that cached feed and compares it with the latest feed.