Students in the United States spend much less time in school than do students in most other industrialized nations, and the school year has been essentially unchanged for more than a century. This is not to say that there is no interest in extending the school year. While there has been little solid evidence that doing so will improve learning outcomes, the idea is often endorsed. Researchers have recently begun to learn more about the effects of time spent on learning from natural experiments around the country. This new body of evidence suggests that extending time in school would in fact likely raise student achievement. In this article, the authors review past research on this issue and then describe the new evidence and the additional insights it provides into the wisdom of increasing instructional time for American students. They also discuss the importance of recognizing the role of instructional time, explicitly, in accountability systems. Whether or not policymakers change the length of the school year for the average American student, differences in instructional time can and do affect school performance as measured by No Child Left Behind. Ignoring this fact results in less-informative accountability systems and lost opportunities for improving learning outcomes. (Contains 3 figures.)