diff --git a/glib/gdatetime.c b/glib/gdatetime.c
index 142338128..0ec390c94 100644
--- a/glib/gdatetime.c
+++ b/glib/gdatetime.c
@@ -3492,12 +3492,12 @@ g_date_time_format_iso8601 (GDateTime *datetime)
   GString *outstr = NULL;
   gchar *main_date = NULL;
   gint64 offset;
-  gchar *format = "%Y-%m-%dT%H:%M:%S";
+  gchar *format = "%C%y-%m-%dT%H:%M:%S";
 
   /* if datetime has sub-second non-zero values below the second precision we
    * should print them as well */
   if (datetime->usec % G_TIME_SPAN_SECOND != 0)
-    format = "%Y-%m-%dT%H:%M:%S.%f";
+    format = "%C%y-%m-%dT%H:%M:%S.%f";
 
   /* Main date and time. */
   main_date = g_date_time_format (datetime, format);