This study analyzed the time dependence of code changes in software projects over their lifetime. The researchers detected foundational periods by establishing time dependence relations between changes. They analyzed how time dependence varied over time in two open source projects, finding that one progressively built on older periods while the other cycled between new and old. Both took over a year to begin strongly relying on past changes. By plotting heatmaps of dependencies between periods, they identified the most foundational periods, which introduced large amounts of code or invasive changes.