Instructional time is a fundamental educational input, yet we have little causal evidence about the effect of longer school days on student achievement. This paper uses a sharp regression discontinuity design to estimate the effects of lengthening the school day for low-performing schools in Florida by exploiting an administrative cutoff for eligibility. Our results indicate significant positive effects of additional literacy instruction on student reading achievement. In particular, we find effects of 0.05 standard deviations of improvement in reading test scores for program assignment in the first year, though long-run effects are difficult to assess.