We propose a verified approach to the formal verification of timed properties using model-checking techniques. We focus on properties commonly found during the analysis of reactive systems, expressed using real-time specification patterns. We use observers in order to transform the verification of these timed patterns into the verification of simpler LTL formulas. While the use of observers for model-checking is quite common, our contribution is original in several ways. First, we define a formal framework to verify that observers are correct and non-intrusive. Second, we define different classes of observers for each pattern and use a pragmatic approach in order to select the most efficient candidate in practice. This approach is implemented in an integrated verification tool chain for the Fiacre language.