NextDate is a function of three variables: month, date, and year. It returns the date of the day after the input date. The month, date, and year variables have integer values subject to these conditions (the year range ending in 2012 is arbitrary, and is from the first edition):
c1. 1 ≤ month ≤ 12
c2. 1 ≤ days ≤ 31
c3. 1812 ≤ years ≤ 2012
As we did with the triangle program, we can make our problem statement more specific. This entails defining responses for invalid values of the input values for the day, month, and year. We can also define responses for invalid combinations of inputs, such as June 31 of any year. If any
of conditions c1, c2, or c3 fails, NextDate produces an output indicating the corresponding variable has an out-of-range value-for example, "Value of month not in the range 1...12." Because numerous invalid day-month-year combinations exist, NextDate collapses these into one message: "Invalid Input Date."