  The reference point for developing any artefact is its specification; to
develop software formally, a formal specification is required. For sequential
programs, pre and post conditions (together with abstract objects) suffice;
rely and guarantee conditions extend the scope of formal development approaches
to tackle concurrency. In addition, real-time systems need ways of both
requiring progress and relating that progress to some notion of time. This
paper extends rely-guarantee ideas to cope with specifications of -- and
assumptions about -- real-time schedulers. Furthermore it shows how the
approach helps identify and specify fault-tolerance aspects of such schedulers
by systematically challenging the assumptions.
