Create a Todo class which has knowledge of start and due dates.

Config.py

+"""
+This module contains some definitions to configure the application.
+"""
+
+TAG_START = 't'
+TAG_DUE = 'due'

Todo.py

+"""
+This module provides the Todo class.
+"""
+
+import datetime
+
+import Config
+import Utils
+import TodoBase
+
+class Todo(TodoBase.TodoBase):
+    """
+    This class adds common functionality with respect to dates to the Todo
+    base class, mainly by interpreting the start and due dates of task.
+    """
+
+    def __init__(self, p_str):
+        TodoBase.TodoBase.__init__(self, p_str)
+
+    def get_date(self, p_tag):
+        """ Given a date tag, return a date object. """
+        string = self.tag_value(p_tag)
+        return Utils.date_string_to_date(string)
+
+    def start_date(self):
+        """ Returns a date object of the todo's start date. """
+        return self.get_date(Config.TAG_START)
+
+    def due_date(self):
+        """ Returns a date object of the todo's due date. """
+        return self.get_date(Config.TAG_DUE)
+
+    def is_active(self):
+        """
+        Returns True when the start date is today or in the past and the
+        task has not yet been completed.
+        """
+        start = self.start_date()
+        return not self.is_completed() and \
+              (not start or start <= datetime.date.today())
+
+    def is_overdue(self):
+        """
+        Returns True when the due date is in the past and the task has not
+        yet been completed.
+        """
+        return not self.is_completed() and self.days_till_due() < 0
+
+    def days_till_due(self):
+        """
+        Returns the number of days till the due date. Returns a negative number
+        of days when the due date is in the past.
+        Returns 0 when the task has no due date.
+        """
+        due = self.due_date()
+        if due:
+            diff = due - datetime.date.today()
+            return diff.days
+        return 0
+
+    def length(self):
+        """
+        Returns the length (in days) of the task, by considering the start date
+        and the due date. Returns 0 when one of these dates are missing.
+        """
+        start = self.start_date()
+        due = self.due_date()
+
+        if start and due:
+            diff = due - start
+            return diff.days
+        else:
+            return 0
+