Time management is essential when simulating multi-agent systems (MASs) as it allows consistent and repeatable simulation runs. So far, time management lacks support to express the timing requirea simulation explicitly and at an abstraction level appropriate for MAS developers. Moreover, integrating time management into a MAS requires the developer to alter the design of the MAS. In this paper, we first propose semantic duration models to capture timing requirements that reflect the semantics of MAS activities in an explicit model. Second, we present a time management infrastructure that starts from a semantic duration model description to integrate all time management functionality into a MAS transparently, i.e. without requiring the developer to alter the design of the MAS. We use aspect-oriented programming technology as it allows separation of concerns, a crucial software engineering requirement. As a case, we apply our approach to the Packet-World.