This paper presents PariSync, a distributed system for clock synchronization in DHT-based peer to peer networks. PariSync is formed by two modules: a topology module, that chooses for each node a small subset of neighbors with which to exchange timing information (piggybacking on the DHT link structure) and an extimation module, that assembles the information into an extimate of the node's offset and drift from a global virtual clock emerging from the consensus of all peers. PariSync works on extremely large peer-to-peer networks (millions of nodes) exhibiting good performance even in the presence of churn and malicious nodes. We provide a version of PariSync in pure Java and in JXTA.