A method for incremental crawling of content stored on a plurality of content providers using aggregation is provided. The method comprises receiving a request to crawl content on one or more associated content providers; retrieving one or more first references to content on a first content provider; retrieving one or more second references to content on one or more second content providers during the same request; aggregating the first and second references; and returning the aggregated first and second references. This is done while taking into consideration opaque timestamp object which is managed in a distributed manner. The opaque timestamp is filled in by the content providers but stored in the crawler side between crawling sessions.