Workflows management systems support the definition and execution of business processes. While business process reeingeneering tools use time information to simulate and optimize processes, the management of time is hardly supported in workflow systems. We introduce a concept for time management for workflow systems. It consists of calculating internal deadlines for all activities within a workflow, checking time constraints and monitoring time at run-time. For the calculation of internal deadlines we extend the netdiagram technique PERT to support the structures usually found in workflows. At run-time this time information is used to pro-acticely avoid time errors and reactively resolve time failures. The concept has been implemented in our prototype workflow management system Panta Rhei.