Hi,
looking at https://en.wikipedia.org/wiki/Growing_degree-day. Your example does not fit the definition. ``If the mean daily temperature is lower than the base temperature then GDD=0.```
The way I understand the definition is: max( (Tmax+Tmin)/2 - Tbase, 0) If the average temperature is below Tbase, then GDD is 0.
In your example, you do (max(Tmax, Tbase) + max(Tmin, Tbase)) ÷ 2 - Tbase and I assume that you take the max of this and 0, to make sure you don't get a <0 GDD.