The horizon of a performance standard is the time given to achieve the standard. Performance standard horizons vary considerably in practice, and the goal-setting literature provides mixed evidence on whether short or long horizons are more effective at eliciting effort from workers. I predict and find that uncertainty in workers’ effort-performance relationship moderates the effect of horizon on effort. Horizon and uncertainty affect expectancy (probability of standard attainment) which in turn determines effort. In an experiment in which the rate of effort needed to achieve the standard was held constant, participants facing low uncertainty decreased effort as I increased horizon. However, those facing high uncertainty increased effort as I increased horizon. These findings are consistent with the expectancy-based predictions, and suggest that managers should jointly determine the level and horizon of performance standards. These findings also provide potential explanations for the variation in horizons observed in practice and for the mixed findings in the goal-setting literature.