diff --git a/pelican/tests/test_utils.py b/pelican/tests/test_utils.py
index 10816199..5fbd066d 100644
--- a/pelican/tests/test_utils.py
+++ b/pelican/tests/test_utils.py
@@ -513,6 +513,9 @@ class TestUtils(LoggedTestCase):
         d = utils.SafeDatetime(2012, 8, 9)
         self.assertEqual(utils.strftime(d, '%-d/%-m/%y'), '9/8/12')
 
+        d = utils.SafeDatetime(2021, 1, 8)
+        self.assertEqual(utils.strftime(d, '%G - %-V - %u'), '2021 - 1 - 5')
+
     # test the output of utils.strftime in a different locale
     # Turkish locale
     @unittest.skipUnless(locale_available('tr_TR.UTF-8') or
diff --git a/pelican/utils.py b/pelican/utils.py
index 8c3a886d..b69cbf95 100644
--- a/pelican/utils.py
+++ b/pelican/utils.py
@@ -50,7 +50,8 @@ def strftime(date, date_format):
     '''
     def strip_zeros(x):
         return x.lstrip('0') or '0'
-    c89_directives = 'aAbBcdfHIjmMpSUwWxXyYzZ%'
+    # includes ISO date parameters added by Python 3.6
+    c89_directives = 'aAbBcdfGHIjmMpSUuVwWxXyYzZ%'
 
     # grab candidate format options
     format_options = '%[-]?.'