Market productivity is often greater, and leisure and other household activities more enjoyable, when people perform them simultaneously. Beyond pointing out the positive externalities of synchronicity, economists have not attempted to identify exogenous causes that affect timing. We develop a theory illustrating conditions under which synchronicity will vary and identify three factors – the amount of daylight, the timing of television programming, and the benefits of coordinating work schedules across a large country – that can alter timing. Using the American Time Use Survey for 2003 and 2004, we first show using a natural experiment that abstracts from the impacts of daylight hours and television timing that an exogenous shock to time in one area leads its residents to alter their work schedules to coordinate more closely with people elsewhere. We then show that both television timing and the benefits of coordinating across time zones in the U.S. generally affect the timing of market work and sleep, the two most time-consuming activities people undertake. These impacts do not, however, differ greatly by people's demographic characteristics, suggesting that longitude and television establish social norms that affect everyone.