What differences a day can make: Quantile regression estimates of the distribution of daily learning gains

•Impact of instructional time varies across achievement distribution.•Differences across distribution are statistically and economically significant.•Results are similar for kindergarten students' math and reading achievement.

Quantile regressions, which exploit quasi-random variation in days between kindergarten students' fall and spring tests, show that the effect of school days on kindergarten students' math and reading gains vary significantly, and monotonically, across the distribution of achievement gains.