A web service has several functional properties (e.g. its operations) and non-functional properties (e.g. quality of service and security parameters). Functional properties usually are static, whereas non-functional properties are often dynamic and thus vary over time. To describe properties with time constraints, the paper introduces WS-TemporalPolicy. WS-TemporalPolicy empowers a service developer to attach a validity period to the properties described in a WS-Policy. The generation, validation, storage and retrieval, and deployment process of temporal policies is supported by a Temporal Policy Runtime Environment. Implementation issues and two use cases are presented to illustrate the use of temporal policies.