  The method of significant moment analysis has been employed to derive
instantaneous schedulability tests for real-time systems. However, the
instantaneous schedulability can only be checked within a finite time window.
On the other hand, worst-case schedulability guarantees schedulability of
systems for infinite time. This paper derives the classical worst-case
schedulability conditions for preemptive periodic systems starting from
instantaneous schedulability, hence unifying the two notions of schedulability.
The results provide a rigorous justification on the critical time instants
being the worst case for scheduling of preemptive periodic systems. The paper
also show that the critical time instant is not the only worst case moments.
