DC has proved to be a promising tool for the specification and verification of functional requirements on the design of hard real-time systems. Many works were devoted to develop effective techniques for checking the models of hard real-time systems against DC specifications. DC model checking theory is still evolving and yet there is no available tools supporting practical verifications due to the high undecidability of calculus and the great complexity of model checking. Present situation of

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... DC model checking is much worse than the one of DC model checking. In view of the results so far achieved, it is desirable to develop approximate model checking techniques for DC and PDC specifications. This work was motivated to develop approximate techniques checking automata models of hard real-time systems for DC and PDC specifications. Unlike previous works which only deal with decidable formulas, we want to develop approximate techniques covering whole DC and PDC formulas. The first results of our work, namely, approximate techniques checking real-time automata models of systems for LDI and PLDI specifications, are described in this paper.