In this chapter, I explain how TimeML, a specification language for the annotation of event-associated temporal expressions in language, was normalized as an ISO international standard, known as ISO-TimeML, with some modifications. ISO’s working group developed TimeML into an ISO standard on event-associated temporal annotation by making four modifications of TimeML: (i) abstract specification of the annotation scheme, (ii) adoption of standoff annotation, (iii) merging of two tags, <EVENT/> and <MAKEINSTNACE/>, to a single tag <EVENT/>, and (iv) treating duration (e.g., two hours) as measurement. Following Bunt’s (2010) proposal for the construction of semantic annotation schemes and his subsequent work, I then formalize ISO-TimeML by presenting a metamodel for its design, an abstract syntax for formulating its specification language in set-theoretic terms, and an XML-based concrete syntax. I also analyze base structures as consisting of two substructures, anchoring and content structures, into the annotation structures of the normalized TimeML.