Memoranda on the Maya Calendars Used in the Books of Chilan Balam

An analytical memorandum compares calendrical reckoning recorded in vernacular Maya chronicles with inscriptional and codical systems, explaining the katun as a 20×360‑day period (7,200 days) and the Maya year‑and‑month cycle of twenty day‑names paired with numbers 1–13. It shows how katuns are named by their opening day and how that nomenclature fixes dates within long cycles, reconciles apparent shifts in month starting days between sources, and computes recurrence intervals — notably that combining katun and year‑month counts yields a least common multiple of 6,832,800 days (360 calendar rounds or 18,720 years) — and uses recorded date examples to illustrate chronological placement.