Synchronous languages have been introduced to provide safety-critical system designers with a formal means of creating trusted designs. The compilers for these languages analyze programs to prove their correctness, and automatically transform them into executable code. In Signal, the compiler has a clock abstraction to support the efficient determination of whether or not program variables occur at any instant. Such information contributes to an assessment of system reactivity and helps optimize the corresponding generated code. In this paper, we first discuss the limitation of this clock abstraction when dealing with non-logical expressions in programs. Then, we propose and implement an interval-based representation,consisting of a generalization of binary decision diagrams, in order to improve the current static analysis in the compiler.