Good planting dates are local. The best guides come from each state's extension service, the people at its land-grant university and county offices who study what grows where, and when.
We read their published planting dates, record each one in our own calendar, and link back to the edition it came from. A state's dates reach your calendar only after we have checked them against the publication and approved that edition. Until then, the calendar shows dates estimated from your average frost dates, and says so.