In this paper, we propose an algebraic language, called Time-AgLOTOS, which is dedicated to express BDI agent plans, according to the features and requirements of Ambient Intelligence (AmI) systems. Plan expressions are written and composed using Time-AgLOTOS, so that plans are built automatically and on the fly, as a system of concurrent processes. This language describes the time-dependent behavior of the agent and provides a theoretical foundation for performing planning under timing constraints, taking into account the duration of actions. Also, it allows to express behavioral capabilities such as communication, mobility and cooperation. In this context, we show how to achieve a powerful mechanism for a contextual guidance based on a specific and formal construction called Contextual Time Planning System (CTPS).