Day and Date Calculations — Definition

Definition

Day-and-date calculation determines a weekday by counting elapsed days from a known date and reducing the difference modulo seven. It combines the weekly cycle with month lengths and the leap-year rule of the calendar being used.

Under the Gregorian rule, century years require divisibility by four hundred to be leap years. The method also depends on whether the target lies before or after the known date and whether the interval includes 29 February.

Counting named dates inclusively is different from counting the one-day steps between two endpoints, so the distinction must be fixed before calculating.