diff --git a/flask/app.py b/flask/app.py
index 83721a6b..ebf4e6a6 100644
--- a/flask/app.py
+++ b/flask/app.py
@@ -41,6 +41,12 @@ from .signals import request_started, request_finished, got_request_exception, \
 _logger_lock = Lock()
 
 
+def _make_timedelta(value):
+    if not isinstance(value, timedelta):
+        return timedelta(seconds=value)
+    return value
+
+
 def setupmethod(f):
     """Wraps a method so that it performs a check in debug mode if the
     first request was already handled.
@@ -177,6 +183,16 @@ class Flask(_PackageBoundObject):
     #: `SESSION_COOKIE_NAME` configuration key.  Defaults to ``'session'``
     session_cookie_name = ConfigAttribute('SESSION_COOKIE_NAME')
 
+    #: A :class:`~datetime.timedelta` which is used to set the expiration
+    #: date of a permanent session.  The default is 31 days which makes a
+    #: permanent session survive for roughly one month.
+    #:
+    #: This attribute can also be configured from the config with the
+    #: `PERMANENT_SESSION_LIFETIME` configuration key.  Defaults to
+    #: ``timedelta(days=31)``
+    permanent_session_lifetime = ConfigAttribute('PERMANENT_SESSION_LIFETIME',
+        get_converter=_make_timedelta)
+
     #: Enable this if you want to use the X-Sendfile feature.  Keep in
     #: mind that the server has to support this.  This only affects files
     #: sent with the :func:`send_file` method.
@@ -486,32 +502,6 @@ class Flask(_PackageBoundObject):
             return rv
         return self.debug
 
-    def _get_permanent_session_lifetime(self):
-        """A :class:`~datetime.timedelta` which is used to set the expiration
-        date of a permanent session.  The default is 31 days which makes a
-        permanent session survive for roughly one month.
-
-        This attribute can also be configured from the config with the
-        `PERMANENT_SESSION_LIFETIME` configuration key.  Defaults to
-        ``timedelta(days=31)``.
-
-        If you want to have this value as seconds you can use ``total_seconds()``::
-
-            app.permanent_session_lifetime.total_seconds()
-
-        Note that the config key can be a timedelta object or number of seconds
-        as integer since Flask 0.8.
-        """
-        rv = self.config['PERMANENT_SESSION_LIFETIME']
-        if not isinstance(rv, timedelta):
-            return timedelta(seconds=rv)
-        return rv
-    def _set_permanent_session_lifetime(self, value):
-        self.config['PERMANENT_SESSION_LIFETIME'] = value
-    permanent_session_lifetime = property(_get_permanent_session_lifetime,
-                                          _set_permanent_session_lifetime)
-    del _get_permanent_session_lifetime, _set_permanent_session_lifetime
-
     @property
     def logger(self):
         """A :class:`logging.Logger` object for this application.  The
diff --git a/flask/config.py b/flask/config.py
index 7b6cd1ee..67dbf9b7 100644
--- a/flask/config.py
+++ b/flask/config.py
@@ -21,13 +21,17 @@ from werkzeug.utils import import_string
 class ConfigAttribute(object):
     """Makes an attribute forward to the config"""
 
-    def __init__(self, name):
+    def __init__(self, name, get_converter=None):
         self.__name__ = name
+        self.get_converter = get_converter
 
     def __get__(self, obj, type=None):
         if obj is None:
             return self
-        return obj.config[self.__name__]
+        rv = obj.config[self.__name__]
+        if self.get_converter is not None:
+            rv = self.get_converter(rv)
+        return rv
 
     def __set__(self, obj, value):
         obj.config[self.__name__] = value
diff --git a/flask/testsuite/config.py b/flask/testsuite/config.py
index 2ed726c8..1f6d79c3 100644
--- a/flask/testsuite/config.py
+++ b/flask/testsuite/config.py
@@ -78,6 +78,11 @@ class ConfigTestCase(FlaskTestCase):
             self.assert_(0, 'expected config')
         self.assert_(not app.config.from_pyfile('missing.cfg', silent=True))
 
+    def test_session_lifetime(self):
+        app = flask.Flask(__name__)
+        app.config['PERMANENT_SESSION_LIFETIME'] = 42
+        self.assert_equal(app.permanent_session_lifetime.total_seconds(), 42)
+
 
 class InstanceTestCase(FlaskTestCase):