I am a member of the DAML Time group, which has been developing an ontology for facts about time, dates, events, and relationships among these things. The version presented here is not quite the consensus version, mainly because I am not sure at this point what the consensus is. However, this version has the advantage that it is expressed completely formally in PDDL (actually, a slight variant called Opt). The notation is basically "parenthesized predicate calculus," and should be readable to anyone familiar with Lisp syntax and first-order logic.
There are two ways to read the ontology. The "walkthrough" contains all of the declarations and axioms, interwoven with an exegesis of the system in the style of literate programming. The following Opt files were generated automatically from the walkthrough, and have been checked by the Opt syntax checker: