The Timor Orogen represents one of the youngest arccontinent collisions exposed on the Earth. It has the potential to provide some of the key parameters about how this style of orogeny evolves. However, the metamorphic age of the highest-grade rocks formed in the collision remains controversial. Using UThPb dating of monazite from amphibolite-grade sillimanite and garnet-bearing schists we show the peak metamorphism occurred at 5.54.7 Ma. The young age of the monazite and the presence of significant amounts of common Pb required the development of a new protocol to simultaneously account for the 230 Th disequilibrium and the 207 Pb common Pb correction. The new estimate of metamorphic age is consistent with the estimates based on plate reconstructions for the initiation of arccontinent collision in East Timor. The metamorphic event is a result of this collision.