Many schools in developing countries have four-hour school days and teach two groups of children each day. Governments are considering lengthening the school day, at great expense, to improve school quality. Advocates of the shift system argue the reform is unnecessary, as evidence from developed countries suggests increasing instructional time only improves achievement scores by small amounts. This paper is the first study of the effect of a large increase in instructional time in a low-income country. In 2005, the Ethiopian federal government directed school districts to abolish teaching in shifts and to lengthen the school day from four to six hours. Districts implemented the reform at different times, creating exogenous variation in instructional time. The author uses a difference-indifference specification controlling for time-invariant unobservables at school level on a unique longitudinal dataset. For 8-year-old children, a longer school day improved writing and mathematics scores, but had no significant effect on reading. However, effects are larger among better-off children: children who are not stunted, children from richer households, and children in urban schools. The exception is that the reform has larger positive effects on girls than boys. The reform thus improves achievement on average, but may exacerbate gaps between wealthier and poorer children.