In this paper the problem of how a robotic agent can create and maintain an ontology in the domain of time is addressed. By an ontology, a mechanism is described that enables a robotic agent to create temporal categories for conceptualizing the world. The creation of a new category is triggered when the agent is unable to temporally distinguish an event from the other events in the context using already adopted categories. In our paper, we firstly give some definitions including definitions for discriminating expressions and for the discrimination game. Next an algorithm is presented for constructing discriminating expressions by stating this problem as a general search problem. And then, it is shown how this algorithm can also be used to create new temporal categories. This is different from most other approaches where ontological categories are defined by humans and the ontologies are fixed in advance.