Spring Planting Planner
Using data from One Yard Revolution on YouTube, this app tells you when to start seeds/plant outside based on your average last frost date.
If you know your last frost date, you can enter it. Otherwise the app uses the Geolocation API along with the Farmsense API and the yacdn.org CORS proxy to get your local last frost date. This is based on 30% probability of hitting 36 degrees F (the highest frost temp).
You can view the app here: https://jimmypautz.com/plants/