Ample evidence documents rising student obesity in summer months and falling student obesity during the school year. One theory for this pattern is that out-of-school days lack some of the structure and health-promoting behaviors that schools provide. Given this observed seasonal pattern, a natural question is whether there is room for policies that alter the summer vacation to serve as an intervention. Compared to traditional calendars, year-round school calendars redistribute the same total number of school days more evenly across the calendar year, reducing the length of the summer break and lengthening breaks elsewhere. Such calendars, therefore, offer the unique opportunity to disentangle whether it is total time in school or the distribution of that time that matters for obesity. Using detailed data for all 5th graders in public schools in California, we take advantage of calendar changes between year-round school calendars and traditional calendars across 20 years to estimate the impacts of school calendar type on obesity. We find evidence of no change in obesity from various year-round school models that redistribute school time. Our findings indicate that there is nothing specific about the particularly long break at summer that matters for obesity accumulation. It is simply time spent in the school environment and not the distribution of that time driving changes in childhood obesity across the year. [The authors acknowledge financial support from the Spanish Ministry of Science and Innovation and the Policy Research Institute at the University of Texas, Austin.]