Use creation date as start date when no explicit start date is given

An explicit start date is given with the t: tag (unless configured
otherwise). Together with the due date, the start date is used to
calculate the length of a todo item (in days).

When no explicit start date is given, consider the creation date as the
start date (if present).

test/test_todo.py

@@ -90,5 +90,29 @@ class TodoTest(TopydoTest):
         todo = Todo("(C) Foo t:2014-01-01 due:2014-01-02")
         self.assertEqual(todo.length(), 1)
 
+    def test_length4(self):
+        todo = Todo("(C) Foo)")
+        self.assertEqual(todo.length(), 0)
+
+    def test_length5(self):
+        todo = Todo("(C) 2015-11-18 Foo)")
+        self.assertEqual(todo.length(), 0)
+
+    def test_length6(self):
+        todo = Todo("(C) 2015-11-18 Foo due:2015-11-19)")
+        self.assertEqual(todo.length(), 1)
+
+    def test_length7(self):
+        todo = Todo("(C) 2015-11-18 Foo due:2015-11-18)")
+        self.assertEqual(todo.length(), 0)
+
+    def test_length8(self):
+        todo = Todo("(C) 2015-11-18 Foo t:2015-11-19 due:2015-11-20)")
+        self.assertEqual(todo.length(), 1)
+
+    def test_length9(self):
+        todo = Todo("(C) 2015-11-18 Foo due:2015-11-16)")
+        self.assertEqual(todo.length(), 0)
+
 if __name__ == '__main__':
     unittest.main()

topydo/lib/Todo.py

@@ -85,9 +85,10 @@ class Todo(TodoBase):
     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.
+        and the due date. When there is no start date, its creation date is
+        used. Returns 0 when one of these dates is missing.
         """
-        start = self.start_date()
+        start = self.start_date() or self.creation_date()
         due = self.due_date()
 
         if start and due and start < due: