Change str of RelativeTime.

common/lib/xmodule/xmodule/fields.py

@@ -207,3 +207,15 @@ class RelativeTime(Field):
             return str(value)
 
         raise TypeError("RelativeTime: cannot convert {!r} to json".format(value))
+
+    def __str__(self):
+        """
+        Makes first H in str representation non-optional.
+
+         str(timedelta) has [H]H:MM:SS format, which is not suitable
+         for front-end (and IsoTime standart), so we forse HH:MM:SS
+         """
+        _str = super(RelativeTime, slef).__str__(self)
+        if len(_str) == 7:
+            _str = '0' + _str
+        return _str

common/lib/xmodule/xmodule/tests/test_fields.py

@@ -167,3 +167,13 @@ class RelativeTimeTest(unittest.TestCase):
 
         with self.assertRaises(TypeError):
             RelativeTimeTest.delta.to_json("123")
+
+    def test_str(self):
+        self.assertEqual(
+            "1:02:03",
+            RelativeTimeTest.delta.to_json(datetime.timedelta(seconds=3723))
+        )
+        self.assertEqual(
+            "11:02:03",
+            RelativeTimeTest.delta.to_json(datetime.timedelta(seconds=39723))
+        )