1. A method for identifying updated data items in a plurality of data items, wherein each data item in said plurality of data items has an associated update time, wherein identifying updated data items comprises identifying those data items in said plurality that have an update time later than a last synchronization time, and wherein said data items are updated such that each data item is associated with one of a plurality of update tokens, each update token being associated with updates performed up to a respective bound period from a time at which said update token is superseded by a successive update token, update tokens being allocated successively in a monotonic sequence, the method comprising: receiving data indicating a last synchronization time; identifying an update token that is associated with one or more updates performed relative to a point in time related to said last synchronization time by said bound period; and identifying data items having an update time after said last synchronization time on the basis of said identified update token.