By means of so-called Scatternets, Bluetooth provides the ability to construct robust wireless multi-hop networks. In this paper we propose a practical protocol for time synchronization of such Bluetooth multi-hop networks. Our protocol makes use of the internal clock maintained by Bluetooth, requires minimal communication overhead, and provides an accuracy of few milliseconds across multiple hops. The protocol has been implemented and evaluated on BTnodes, an embedded computing platform which uses Bluetooth for ad hoc networking.