SOURCE: “Solving the Riddle of Time,” in Calendar: Humanity's Epic Struggle to Determine a True and Accurate Year, Avon Books, Inc., 1998, pp. 187-208.

In the following essay, Duncan recounts the efforts of those involved in the Gregorian calendar reform, the technical difficulties they faced, and the reaction to their work.

The patriarch has also subscribed to our calendar and admitted that it is very good. I hope that it will soon be published, because the Pope is quite eager.

—Christopher Clavius, 1581

None of the three men responsible for fixing the calendar was a conqueror, notorious lover, heretic, or lone monk pondering the cosmos from a cell in a monastery. They were not even particularly flamboyant, and certainly not free thinkers in the spirit of a Bacon or even a Paul of Middelburg—all of which might account for their success.

They included an obscure physician from the toe...