The notion of contracts (or agreements) has been considered useful to promote mutual understanding and configuration in service provision between its consumer and provider. However, it has not been discussed how consistency can be ensured in multiple contracts that a composite service establishes with its component service providers and its consumers. In response to the problem, this study provides a theoretical foundation, based on Event Calculus, for expressing and reasoning about various constraints on contracts as well as consistency properties. On the basis of the proposed foundation, this study also discusses how an existing reasoner on Event Calculus can be used to help development of consistent contractual policies, or constraints on contracts. A tool is developed that wraps the reasoner and facilitates consistency checking and testing.