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The schedulability analysis of real-time systems (even on a single processor) is a very difficult task, which becomes even more complex (or undecidable) when periods or deadlines become uncertain. In this work, we propose a unified formalism to model monoprocessor schedulability problems with several types of tasks (periodic, sporadic, or more complex), most types of schedulers (including , and ), with or without preemption, in the presence of uncertain timing constants. Although the general case is undecidable, we exhibit a large decidable subclass. We demonstrate the expressive power of our formalism on several examples, allowing also for robust schedulability.