Evidence of mysterious 'recurring nova' that could reappear in 2024 found in medieval manuscript from 1217

Sep 6, 2020
"dramatically increases in brightness for about a week every 80 years. But it has been scientifically documented only twice — once in 1866, and again in 1946. (The star’s next long-awaited flare-up is expected in 2024)."

Am I being stupid here? Why does it jump to 78 years to be expected 2024?