Time synchronization is a critical piece of infrastructure for any distributed system. Distributed, wireless sensor net-works make extensive use of synchronized time, but often have unique requirements in the scope, lifetime and precision of the synchronization achieved, as well as the time and energy required to achieve it. Existing time synchronization methods need to be extended to meet these new needs. We outline the synchronization requirements of future sensor networks and present an implementation of our own low-power synchronization scheme, post-facto synchronization. We also describe an experiment that characterizes its performance for creating short-lived and localized but high-precision synchronization using very little energy.