 This idea of adding a temporal aspect makes sense not only for knowledge, but also for other modal concepts. For example, one can think of adding a temporal feature to a modal system for preferences, thus discussing different ways in which the preferences of an agent might change over time (and also why they do so, if further dynamic machinery is added to talk about actions and their consequences; Grüne-Yanoff &amp; Hansson 2009; Liu 2011). Similarly, adding a temporal aspect to modal systems of  deontic logic  raises interesting concepts and questions, an example being the notion of deontic deadlines, discussed in  section 4.7 on  obligations and time.