How do calendars and clocks influence considerations of school effectiveness? From the creation of compulsory education to the future of virtual schooling, Weiss and Brown trace two centuries of school practices, policies and research linking the concept of time with 'opportunity to learn'. School calendars and clocks are shaped by both the physical and social worlds, and the 'clock of schooling' is shown to be one of the 'great clocks of society' that helps to frame school effectiveness.