Policies that aim at improving student achievement frequently increase instructional time, for example by means of an extended day program. There is, however, hardly any evidence that these programs are effective, and the few studies that allow causal inference indicate that we should expect neutral to small effects of such programs. This study conducts a randomized field experiment to estimate the effect of an extended day program in seven Dutch elementary schools on math and language achievement. The empirical results show that this three-month program had no significant effect on math or language achievement. (author abstract)