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.

中文翻译：

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从开学日起就解开学年的影响：来自TIMSS评估的证据

学校通常必须在延长学年或上学日之间做出决定。本文通过一种方法框架来检验教学时间分布的变化对八年级学生成绩的影响，该方法框架将每年的总教学时间分解为每年的天数和每天的小时数。这项研究的数据集汇集了来自80个国家的近900,000名学生的观察结果以及四个国际数学和科学研究趋势（TIMSS）评估（1995-2007）的四年期测试周期。我发现教学时间对学生成绩的积极影响主要是由上学时间的长短而不是学年的长短驱动的，前者的边际收益递减。社会经济地位低下的学生最有可能从更长的上课时间中获得收益。此外，将课余时间与在TIMSS测试的科目上花费的教学时间隔离开来，可以发现从未测试的科目上花费的时间有溢出效应，这对贫困学生特别有意义。相比之下，在测试组中花费的时间影响在各个学生群体中更为均匀。