Cooling Degree Days seem to have varying definitions (from other references):

Cooling degree days (CDDs) are a widely used measure of heat exposure and cooling demand. It measures how much the mean temperature exceeds a reference temperature each day over a given period, using the dry bulb temperature. For example, considering 18ºC as the baseline temperature for CDD calculation, a day with a mean outdoor temperature of 30°C, has 12 CDDs (30-18). If the next day has a mean temperature of 28°C, it has 10 CDDs. The total for the two days is, therefore, 22 CDDs.

Degree days are measures of how cold or warm a location is. A degree day compares the mean (the average of the high and low) outdoor temperatures recorded for a location to a standard temperature, usually 65° Fahrenheit (F) in the United States. The more extreme the outside temperature, the higher the number of degree days. A high number of degree days generally results in higher levels of energy use for space heating or cooling.

So this all seems quite arbitrary. As everything warms I suspect that we'll be adjusting that base temp from 18'C/65'F to something warmer that is still tolerable. In the summer I find 24'C indoor temps to be just fine as long as humidity doesn't get too high.