The Zotero timeline allows you to visualize the items in a Zotero collection on a chronological basis, using either the publication date or the date you added the item to Zotero.
To create a timeline, go to the Tools menu, and select "Create Timeline". A timeline will be created for the collection which is open on your desk. You can navigate laterally in the timeline, change the defaut option "date published" to "date added" or "date modified", or highlight a specific search term in one of the 4 colors provided.
This screencast allows you to better understand how the timeline works.