In this paper an extension of SDL by real-time assertions is suggested. A denotational compositional semantics in terms of timed high level Petri nets is given. This semantics allows to reason about timed SDL-specifications by simulation of the underlying high level nets as well as by unfolding these into timed low level nets and then applying analysis methods for timed Petri nets or temporal model checking. The approach is embedded in the MOBY-tool