Hmmmmm, perhaps your query was not so much about corn as whether the seed packets always had the best information? For the most part, I think the seed packets do pretty well describing the culture of the cultivar in a few words. Here is the problem; every garden has its own mini environment and the climate varies dramatically across the country. What works for me may not work for you, and especially when talking about planting times. Gardening practices/methods also dictate different spacings and planting times.
I suggest you check with your local extension, local gardeners, and garden stores in your area. Gardeners in your area can give you much better advice than someone clear across the country. That is the nice thing about this forum, there are people from all over checking in. You may get answers from people in your area.
Gardening at 5000 feet elevation, zone 4/5 Northern Utah, Frost free from May 25 to September 8 +/-