Policymakers have renewed calls for expanding instructional time in the wake of the COVID-19 pandemic. We establish a set of empirical facts about time in school, synthesize the literature on the causal effects of instructional time, and conduct a case study of time use in an urban district. On average, instructional time in U.S. public schools is comparable to most high-income countries, with longer days but shorter years. However, instructional time varies widely across U.S. public schools with a 90th-10th percentile difference of 190 total hours. Empirical literature confirms that additional time can increase student achievement, but how this time is structured matters. Our case study suggests schools might also recover substantial lost learning time within the existing school day.