Through a synthesis of test publisher norms and national longitudinal data sets, this study provides new national norms of academic growth in K-12 reading and math to help reinterpret conventional effect sizes in time units. We propose "d'," a time-indexed-effect-size metric to estimate how long it would take for an "untreated" control group to reach the treatment group outcome in terms familiar to educators--years/months of schooling. It serves as a supplement to conventional effect-size metrics, such as Cohen's "d'," by taking into account different amounts of time needed for learning at different ages or grade levels. Through applications to Project STAR small class effects and NAEP racial achievement gaps, we demonstrate how to interpret and use "d'." It is expected to provide a more developmentally appropriate context for interpreting the size of an effect, a step toward bridging the gap between educational research and practice.