New analysis has identified how the design of Stonehenge may have represented a calendar, helping people track a solar year of 365.25 days calibrated by the alignment of the solstices. Although it had long been thought to be a calendar, pinpointing how it functioned was only possible thanks to modern discoveries. The large sarsens that dominate the site appear to reflect a calendar with 12 months of 30 days, divided into 10 day ‘weeks’. An intercalary month and leap days aligned it with the solar year. Such calendars had been developed in ancient Egypt, raising the possibility Stonehenge’s calendar system had its roots elsewhere.