While instructional time is viewed as crucial, little is known about the effectiveness of reducing absences relative to increasing the number of school days. This paper constitutes the first attempt to jointly estimate the relative effectiveness of reducing absences to extending the school calendar on test score performance. Using administrative data for North Carolina public schools, we exploit a state policy that provides variation in the number of days prior to standardized testing and find substantial differences between these effects. Extending the school calendar by ten days increases math and reading test scores by only 0.8% and 0.2% of a standard deviation, respectively; a similar reduction in absences would lead to gains of 5.8\% and 3\% in math and reading. Our findings indicate substantial heterogeneity across student ability, suggesting that targeting absenteeism among low performing students would aid in narrowing current gaps in performance. Finally, we analyze whether different institutional settings could affect how school administrators and teachers respond to possible extensions of the school calendar. Our findings suggest that low performing schools value an extra day of class more when monetary bonuses are in place.