The app is based on the data supplied by the National Climatic Data Center and I'm using the 50% chance numbers (meaning that there is still a 50% chance of a frost after the dates given). This is why the app gives you a disclaimer to keep an eye on your forecast when planning your transplants.
Having said that, I am considering moving all the dates forward to the 80% mark to make things a little safer for the newbies who don't have your knowledge and experience.
