diff --git a/qubesmanager/settings.py b/qubesmanager/settings.py
index da074c9d..c03cbcfd 100644
--- a/qubesmanager/settings.py
+++ b/qubesmanager/settings.py
@@ -1465,7 +1465,7 @@ def set_fw_model(self, model):
         if model.temp_full_access_expire_time:
             self.temp_full_access.setChecked(True)
             expire_time = model.temp_full_access_expire_time - \
-                          firewall.datetime.datetime.now().strftime("%s")
+                          firewall.datetime.datetime.now().timestamp()
             self.temp_full_access_time.setValue(int(expire_time / 60))
 
     def disable_all_fw_conf(self):