The UML Profile for Modeling and Analysis of Real-Time and Embedded (MARTE) systems has recently been adopted by the OMG. Its Time Model extends the informal and simplistic Simple Time package proposed by UML2 and offers a broad range of capabilities required to model real-time systems. The MARTE OMG specification introduces a Time Structure inspired from Time models of the concurrency theory and proposes a new clock constraint specification language (CCSL) to specify, within the context of UML, logical and chronometric time constraints. This report specifies the syntax and a formal semantics of a subset of CCSL, called kernel CCSL. This semantics is to be the reference semantics of CCSL.