diff --git a/mellon/utils.py b/mellon/utils.py
index 992395a..1c8ff16 100644
--- a/mellon/utils.py
+++ b/mellon/utils.py
@@ -9,6 +9,7 @@ import requests
 
 from django.core.urlresolvers import reverse
 from django.template.loader import render_to_string
+from django.utils.timezone import make_aware, utc, now
 import lasso
 
 from . import app_settings
@@ -102,6 +103,7 @@ def get_idps():
                 yield idp
 
 def flatten_datetime(d):
+    d = d.copy()
     for key, value in d.iteritems():
         if isinstance(value, datetime.datetime):
             d[key] = value.isoformat() + 'Z'
@@ -116,7 +118,10 @@ def iso8601_to_datetime(date_string):
     if not m:
         raise ValueError('Invalid ISO8601 date')
     tm = time.strptime(m.group(1)+'Z', "%Y-%m-%dT%H:%M:%SZ")
-    return datetime.datetime.fromtimestamp(time.mktime(tm))
+    return make_aware(datetime.datetime.fromtimestamp(time.mktime(tm)), utc)
+
+def get_seconds_expiry(datetime_expiry):
+    return (datetime_expiry - now()).total_seconds()
 
 def to_list(func):
     @wraps(func)
diff --git a/mellon/views.py b/mellon/views.py
index a319455..881d3ec 100644
--- a/mellon/views.py
+++ b/mellon/views.py
@@ -133,7 +133,7 @@ class LoginView(LogMixin, View):
                         unicode(user), attributes['name_id_content'])
                 request.session['mellon_session'] = utils.flatten_datetime(attributes)
                 if 'session_not_on_or_after' in attributes:
-                    request.session.set_expiry(attributes['session_not_on_or_after'])
+                    request.session.set_expiry(utils.get_seconds_expiry(attributes['session_not_on_or_after']))
             else:
                 return render(request, 'mellon/inactive_user.html', {
                     'user': user,