Degree days are used as an approximate measure of energy demand to let utilities and householders keep track of what energy use for heating or cooling ought to be from day to day or year to year.
To calculate degree days for a given day, add the high and low temperatures, divide by two and compare that value with 65 degrees Fahrenheit.
If the value is 60, there were five heating degree days. If the value is 70, there were five cooling degree days.
The only time there are zero degree days is when the daily average temperature is 65 degrees, because the same theory that says you turn on the furnace when the temperature is below 65 degrees says you turn on the air conditioner when it is above 65 degrees.