scheduled and due dates for tasks

[pcause]

For tasks, it is usually good to have scheduled and due dates. Scheduled is wen one is supposed to start work and due is when it should be done. One can have either or both. Then a feature to arrange nodes or items withing a node by either date allows a user to view their work in time order. In todo.txt they use t:yyy-mm-dd for start/scheduled and due:yyy-mm-dd for these dates.