1582 reform by Pope Gregory XIII. Patched the Julian leap rule by adding 'except divisible by 100, unless also divisible by 400', giving an average 365.2425-day year (still drifts slightly). Dropped 10 accumulated drift days by skipping October 5–14 so a Thursday was followed directly by a Friday. Allowed a 120-year transition window during which Julian and Gregorian leap years would coincide. Adoption stretched over 400+ years: Germany, Italy, Poland, Portugal and Spain on day one; France in 1805; Saudi Arabia in 2016; some countries flipped back and forth between calendars; German and Swiss cities within the same country adopted 200 years apart. Used in the talk as a masterclass in deploying breaking changes against a 1600-year-old customer base: simple patch instructions on top of the old rule, fixing drift while at it, picking a day with minimal weekly disruption, and a multi-generation adoption window.