HDDs and CDDs are "additive", that is, they may be validly summed or averaged over days, weeks, seasons, etc. to provide comparative data.
Using HDDs and CDDs for a "Region" will facilitate the ability to understand the relationship between Degree Days and energy costs / carbon emissions.
When calculating Heating Degree Days and Cooling Degree Days, one or the other value will always be zero. In the rare case that the Temperature Average exactly equals the Degree Day Reference Point, both the Heating Degree Days value and the Cooling Degree Days value will equal zero.