An activity planning system comprises a knowledge base, a query processor, and a temporal reasoner. A query including temporal constraints is input into the query processor. The query processor converts the query into a formal representation. The formal representation is a formal graphical semantic representation grounded on an ontology defined in the knowledge base. The temporal reasoner processes the query representation output by the query processor against the knowledge base which defines a set of object. For each object, the temporal reasoner produces a normalized score from 0 to 1 to indicate the degree of how likely the object satisfies the temporal constraints imposed by the query.