diff --git a/pelican/utils.py b/pelican/utils.py
index 8c3a886d..7b369b08 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 = '%[-]?.'
@@ -98,7 +99,6 @@ class SafeDatetime(datetime.datetime):
         else:
             return super().strftime(fmt)
 
-
 class DateFormatter:
     '''A date formatter object used as a jinja filter