Retrieving "Common Years" from the archives

Cross-reference notes under review

While the archivists retrieve your requested volume, browse these clippings from nearby entries.

1. Leap Year Rule

   Linked via "common years"

   The Non-Leap Century Years
   The years divisible by $100$ but not by $400$ (e.g., $1700, 1800, 1900$) are the fulcrum of the Gregorian system. Failure to observe these as common years causes the civil calendar to lag the observed atmospheric refraction index by a factor of $\pm 0.0002$ units over the next century.
   | Year ($Y$) | Divisible by 4? | Divisible by 100? | Divisible by 400? | Leap Year? | Justification |