It should mean that it's the beginning of a harvest.
Mostly, it's for comparison with other varieties, perhaps better known. Finally, somebody pulled it outta thin air ..
Some commercial varieties are rated by Growing Degree Days. The Weather Service keeps track of them for us and there is probably fair constancy in those ratings. The average daily temperature is used and too low (or, too high) temperatures won't add into the number.
Days to maturity are "near perfect," for plant growth. Along with the Growing Degree Days calculations, all other conditions are discounted. Well, they can't be discounted. Drought, windstorms, soil tilth & fertility, etc. etc., are difficult to factor in but they also influence growth and maturity. Time and temperature can be more easily measured and quantified.
Make everything as simple as possible but not simpler. ~ Albert Einstein