International studies suggest that the U.S. ranks below many Asian and European countries in the 21st century in terms of mathematics and science achievement. Few have looked beyond the classroom to understand these differences. Absolute and relative time spent in various out-of-school time (OST) activities may provide one explanation. This study used the Trends in International Mathematics and Science Study (TIMSS), which includes data on the OST and achievement of 8th graders from nearly 50 countries worldwide. OST variables included technology-based (e.g., using the internet), labor (e.g., chores), and leisure activities (e.g., sports, playing with friends). Students completed an internationally standardized mathematics and science achievement test. Results for absolute OST suggest that, beyond the large contribution of a country's human development index, OST is an important predictor of achievement. Further, relative OST is an important predictor, such that, those countries whose profile of time use was highest in technology also had the highest achievement scores. Future research should consider a broader view of education and related contexts that includes understanding the variability in OST use both within and between nations.