Educational reformers use international evidence to argue that increasing the number of days in school and the length of the school day will improve academic achievement. However, the international data used to support these claims (1999 Third International Math and Science Survey and 2000 Program for International Student Assessment) show no correlation between time in school and achievement. In this article, the author re-examines the effects of instructional time using improved measures of instructional time, a more extensive data set (2006 Program for International Student Assessment), and a more nuanced multilevel model. The author finds mixed evidence of a positive effect of subject-specific instructional time on achievement, controlling for socioeconomic status, school characteristics, and country-level traits. The author finds no effect of the length of the school year on academic achievement and that sample selection and the specific uses of time in school have a strong influence on conclusions about the effectiveness of instructional time.