Quantitative temporal constraints are an essential requirement for many planning domains. The HTN planning paradigm has proven to be better suited than other approaches to many applications. To date, however, efficiently integrating temporal reasoning with HTN planning has been little explored. This paper describes a means to exploit the structure of a HTN plan in performing temporal propagation on an associated Simple Temporal Network. By exploiting the natural restriction on permitted temporal constraints, the time complexity of propagation can be sharply reduced, while completeness of the inference is maintained. Empirical results indicate an order of magnitude improvement on real-world plans.