Expanded learning time (ELT) is a practice that entails increasing the length of the school day for all students, with the goal of improving student outcomes, especially in low-performing schools. Boston Public Schools (BPS) has been implementing ELT as an improvement strategy in low-performing schools since 2006. In 2015 the district drastically increased the number of participating schools, with a plan of expanding the day in 60 elementary, middle, and K-8 schools by 2018. How much additional time is added to the school day and parameters for using the extra time depend on the ELT funding source, but, in general, schools have wide latitude for program implementation. BPS collaborated with American Institutes for Research (AIR) on IES-funded grant R305H150013 to study its ELT program types, with the goal of understanding the impact of ELT on student academic outcomes. Looking at whether additional time alone affects student outcomes, researchers performed comparative interrupted time series (CITS) analyses to examine impacts on student performance in the first two years after ELT implementation. A statistically positive impact was found on ELA and mathematics achievement in the first and second year following ELT implementation. The impact was especially strong in mathematics (both in magnitude and statistical significance). These results suggest that by extending their school days, ELT schools in BPS improved student achievement in these subjects. Analyses of student demographic subgroups showed evidence of impact gains for Black and Hispanic students in mathematics and ELA during both Year 1 and Year 2 ELT implementation. Some gains were also observed for economically disadvantaged students and ELLs, primarily in Year 2 of ELT implementation. No impact was found for White or Asian students or students with disabilities. To further clarify how schools with longer days in BPS used their time, BPS staff members contacted ELT schools in October 2016 to provide total minutes per week spent on specific activities during the previous school year (2015-16). In general, ELT activities were more likely to be positively correlated with higher CPI scores in mathematics than in ELA, even when the activities the school implemented were focused on ELA. These academic achievement findings indicate that ELT is having a positive impact on student achievement in the district. The results are especially positive for Black and Hispanic students, who make up a large percentage of the student population in many Boston schools. Overall, results have been most promising for mathematics. This was also seen in the descriptive results, in which ELT schools that integrated intervention time into their day see more growth in their students' mathematics performance than schools that have not used ELT to add intervention time. This is true both for schools with intervention time focused on ELA and/or mathematics: results appear to be most promising for mathematics achievement. [For the Year 1 Findings Report, see ED621605.]