Time management is essential when simulating multi-agent systems (MASs) as it allows consistent and repeatable simulation runs. So far, time management lacks explicit support to express the timing characteristics of a simulation at the level of activities within a MAS. Moreover, integrating a MAS into a simulation platform requires the developer to alter the design of a MAS. In this paper, we first propose semantic duration models to capture timing characteristics that reflect the semantics of MAS activities in an explicit model. Second, we describe a simulation platform prototype that supports semantic duration models and integrates time management into a MAS without requiring the developer to alter the design of a MAS. We employ aspect-oriented programming technology as it allows separation of concerns, a crucial software engineering requirement. We use the Packet World as a case to illustrate our approach