We propose a formal approach for the definition of domain-specific modelling languages (DSMLs). The approach uses basic Model-Driven Engineering artifacts for defining a DSML's syntax (using metamodels) and its operational semantics (using model transformations). We give formal meanings to these artifacts by mapping them to the K semantic framework. The mapping is implemented in the Rascal metaprogramming language. Since the resulting K definitions are executable, one obtains an execution engine for DSMLs and gains acces to K's formal analysis tools. We illustrate the approach on xSPEM, a language for describing the execution of tasks constrained by time, precedence, and resources.