Hi @Man
If the behaviour is configured to be executed only when the date changes you have a method available to obtain the information of the updated field.
def dueDate = getFieldById(getFieldChanged())
With this line you obtain the field that has been updated, and you are able to obtain the value using
def selectedOption = dueDate.getValue() // Here be careful with the variable types.
after that, you will have the new value for due date, and you can compare it with the value stored in the underlyingIssue variable.