Christian Zeller's 1882 reformulation of the calendar: move the start of the year to March so that February — the month that breaks everything because of the Romans — sits at the very end. All calendar math is then done in this simpler model, and the result is translated back at the very end. A fundamental example of rephrasing a problem in convenient coordinates (like switching between polar and Cartesian), and a key building block of the Neri–Schneider algorithm.