Each concept here is mapped to its prerequisites — the ideas you'd need first to understand it — all the way down to four foundations: Space, Time, Energy, Pattern. Click any prerequisite to drill down, or scroll for the chain graph.
A calendar is a system for organizing days and months, which relies on the concept of measurement to quantify duration, allowing us to assign numerical values to the passage of time.
External reference — all senses of the word “calendar” on Wiktionary. This atlas concept maps to only the slice of meaning relevant to the prerequisite graph.