Compare your growing season
Would you like to compare your growing season with other locations around the region and around the US? (see link below)
Don't worry too much about the term "Growing Degree Days" unless you want to explore that subject. It would make as much sense to call them "units" as "days" and some people do. You can just use the numbers for purposes of comparison.
Using corn as a reference crop is simply a standard for a warm-season crop. Probably tomatoes could be substituted since they have similar heat requirements. And, for comparative purposes, once again, it doesn't really make much difference when one is considering the growing season.
Using a cool-weather crop like peas would make a difference since growth would take place at a reasonable rate at in cooler climates but, we can pretty much grow peas in any American garden at some time of the year. What we may have difficulty growing are long-season crops, not short ones.
And, for growing something like corn or tomatoes - take a look at some of the coastal cities. Can you imagine living in Eureka, California! Well, I can since I attended high school about 8 miles away. And, as a comparison consider Anchorage, Alaska. Ha, ha, ha . . . better gardening in Alaska than California!! Yep, I bet it usually is. (David, we were talking about day-length awhile ago and here's your evidence.)
I have no idea why all the data and cities aren't included. Why, for instance Mt. Washington, New Hampshire is there (sure, that's where they grow LOTS of corn! ;o) and why Denver's numbers aren't all included.
Here's where you need to go to find more info on Denver and other Colorado cities - scroll down the left-side until you can click on 1st the city and then "Growing Degree Days." Here's the map of the West for other states.
Here is a link that might be useful: Current year-to-date GDDs for selected US cities