This study (an impact evaluation of the Chilean full school day program) uses difference-in-differences to estimate the effect of a large increase in instructional time on high school students' academic achievement. The main findings are (i) the program had a positive effect on students' achievement in both mathematics and language; (ii) the effect-size on language achievement was 0.05-0.07 standard deviations and not sensitive to control for covariates, different control groups, and historical trends; (iii) the effect on mathematics achievement was not sensitive to control for covariates, but was sensitive to use different control groups, and historical trends; the effect-size on mathematics achievement ranged from 0.00 to 0.12 standard deviations; and (iv) the program effect has been constant over time. Finally, there is evidence suggesting that the program had larger positive effects on rural students, students who attended public schools, and students situated in the upper part of the achievement distribution. (Contains 1 figure and 6 tables.)