There are instances of a content item having synchronization information, one being an instance of a reference content item, another being a candidate content item. A synchronizing processor determines a synchronization point between the reference content item and the candidate content item, on the basis of matching the synchronization information of the reference content item and the candidate content item. A timeline metadata transmitter transmits stored timeline metadata to a device on which the reference content item is playing, the transmission of the stored timeline metadata beginning from the synchronization point. The stored timeline metadata includes timeline metadata that has been previously aggregated from earlier broadcasts of instances of the reference content item. Related systems, apparatus, and methods are also described.