We have designed and implemented a new programming language for hard real-time systems. Critical timing constraints are specified within the language, and ensured by the compiler. The main novel feature of the language is that programs are extensible in two dimensions without changing their timing behavior: new program modules can be added, and individual program task can be refined. The mechanism that supports time invariance under parallel composition is that different program modules communicate at specified instances of time. Time invariance under refinement is achieved by conservative scheduling of the top level. The language, which assembles real-time tasks within a hierarchical module structure with timing constraints, is called Hierarchical Timing Language (HTL). It is a coordination language, in that individual tasks can be implemented in other languages. We present a distributed HTL implementation of an automotive steer-by-wire controller as a case study.