Schools often have to decide between extending the length of the school year or the school day. This paper examines the effects of changes in the distribution of instructional time on eighth-grade student achievement through a methodological framework that disaggregates total yearly instructional time into separate inputs for days per year and hours per day. This study's dataset brings together nearly 900,000 student observations across eighty countries and four quadrennial testing cycles of the Trends in International Mathematics and Science Study (TIMSS) Assessments (1995-2007). I find that the positive effects of instructional time on student achievement are driven largely by the length of the school day and not by the length of the school year, with diminishing marginal returns to the former. Socioeconomically underprivileged students are most likely to realize gains from a longer school day. Furthermore, isolating the amount of instructional time spent on TIMSS-tested subjects from the rest of the school day reveals spillover effects from time spent in non-tested subjects that are especially meaningful for underprivileged students. In contrast, the effects of time spent in tested subjects are more homogeneous across student groups.