“The Times They Are A-Changing” (B. Dylan), and with them the structures, schemas, master data, etc. of data warehouses. For the correct treatment of such changes in OLAP queries the orthogonality assumption of star schemas has to be abandoned. We propose the COMET model which allows to represent not only changes of transaction data, as usual in data warehouses, but also of schema, and structure data. The COMET model can then be used as basis of OLAP tools which are aware of structural changes and permit correct query results spanning multiple periods and thus different versions of dimension data. In this paper we present the COMET metamodel in detail with all necessary integrity constraints and show how the intervals of structural stabilities can be computed for all components of a data warehouse.