The issue is related to the option of adding recurring bills, which a user can do by changing the Repeat field to something other than Once (i.e. Monthly, Weekly). This will enable the End Date field, a customized widget that uses a gtk.Calendar to allow the user to define how far into the future to automatically create instances of the bill being entered. I want to be able to define a start and end date for the catalog, so that the user doesn't pick a date which is prior to the Due Date. In other words, if the due date is tomorrow, don't allow the user to set the end date to yesterday. A couple of use cases that have to be taken into consideration are:

    * When adding a new bill, the end date can not be earlier/less than the due date; therefore, every time the user changes the due date, the end date should be automatically updated to be at least equals to the due date;
    * If the end date and due date are equal and the user chose Monthly or Weekly for the Repeat field, a dialog should pop up alerting that only one instance will be created;
    * If editing an existing bill, the Repeat and End Date are disabled; however, changing the Due Date should still update the value for End Date (for cosmetic reasons);