Type a place (or coordinates), pick the period and a base temperature, and get the accumulated growing degree days from real station-model weather data.
Daily GDD = max(0, (Tmax + Tmin)/2 − Tbase), summed over the period — with the standard agronomic option of capping Tmax at 30 °C (the modified method used for maize). Weather comes from Open-Meteo's free archive API, fetched directly by your browser — your location query goes to Open-Meteo, not to this site, and nothing is stored here.