Our goal is always to create apps that are intuitive and easy to use to the point where too much documentation is not necessary. However, you can get help with the accepted due date formats, by tapping on "More..." when you move the cursor to the date field.
Furthermore, when using the app for the first time, pretty much every function is explained through our tooltips that appear as you navigate through the app until you tap on them. If you've missed them, you can reinstall the app and all will be reactivated.
If you have any issues with any features in particular, please let us know.