The main purpose of this paper is to present a new object oriented data model for structured documents and their temporal features. We first characterise historical documents as those participating into a set of applications where temporal aspects of documents are specially relevant. Then, we describe a formal temporal data model for historical documents, which relies on two specially suited type systems. One expresses the flexible structure of document classes. The other allows defining metadata. In this way, metadata becomes a central feature, which is introduced into the data model by means of metaclasses. Finally, we discuss which are the novel uses of metadata in a digital library of historical documents.