I agree that "it's only an average". Within one seed catalog, if that company trials most of the seeds they sell, they should be consistent with the exact microclimate in that vendor's trial spot.
Otherwise, you have to look at the other varieties
in the same seed catalog, and guess that the middle numbers are "mid-season" varieties. Then you can guess that the ones with shorter DTM will mature that many days faster than THEIR middle numbers will be around that much faster in your yard, compared to YOUR middle numbers.
For species where "warmer means faster", like tomatoes, it might be possible to give more meaningful information using accumulated "degree days" or "heat days" - adding up the number of degrees above 50 F (or some other base temperature).
For example, if the mean temperature one day was 51F, add 1. If the avg temp was 60, add 10. ("Mean" being taken as daily max minus daily min over 2 ... not and hourly average or anything fancy.)
But no one does that! And of course it fails when the temperature gets hot enough to stress the plants.
http://www.weather.com/outdoor...
