Book of Valuable Recipes

Church & Dwight's 1915 recipe book for the great Arm & Hammer soda — which also prints a perpetual calendar, because why not.

The Perpetual Calendar, Made to Work

Among the recipes and remedies, the book prints a perpetual calendar — a table of rules for finding the weekday of any date. Here is the same promise kept by machinery: choose any month of any year.

1583 is the first full year of the Gregorian calendar — before that, the book's own table wouldn't agree either.