We propose a verified approach to the formal verification of timed properties using model-checking techniques. We focus on properties expressed using real-time specification patterns, which can be viewed as a subset of timed temporal logics that includes properties commonly found during the analysis of reactive systems. Our model-checking approach is based on the use of observers in order to transform the verification of 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 our 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.