Methods, systems, and apparatus, including computer programs encoded on a computer storage medium, for identifying a plurality of items to be displayed to a user, each item including digital content and being associated with a respective timestamp, receiving a plurality of scores, each score being associated with an item of the plurality of items, determining a subset of items to be displayed to the user based on the plurality of scores, the subset of items including items of the plurality of items, grouping items in the subset of items based on respective timestamps to provide a plurality of item groups, for each item group, determining an order of items based on respective scores, and transmitting instructions to display items to the user based on the plurality of item groups and in the order within each item group.