diff --git a/gtk/gtkcalendar.c b/gtk/gtkcalendar.c
index 5bbb6cac19..a93d92e384 100644
--- a/gtk/gtkcalendar.c
+++ b/gtk/gtkcalendar.c
@@ -687,7 +687,7 @@ gtk_calendar_init (GtkCalendar *calendar)
       {
 #ifndef G_OS_WIN32
         tmp_time= (i+3)*86400;
-        strftime ( buffer, sizeof (buffer), "%a", gmtime (&tmp_time));
+        strftime (buffer, sizeof (buffer), "%a", gmtime (&tmp_time));
         default_abbreviated_dayname[i] = g_locale_to_utf8 (buffer, -1, NULL, NULL, NULL);
 #else
         if (!GetLocaleInfoW (GetThreadLocale (), LOCALE_SABBREVDAYNAME1 + (i+6)%7,
@@ -703,7 +703,8 @@ gtk_calendar_init (GtkCalendar *calendar)
       {
 #ifndef G_OS_WIN32
         tmp_time=i*2764800;
-        strftime ( buffer, sizeof (buffer), "%B", gmtime (&tmp_time));
+        /* Note: "%OB" works only in glibc >= 2.27 and in BSD and OS X.  */
+        strftime (buffer, sizeof (buffer), "%OB", gmtime (&tmp_time));
         default_monthname[i] = g_locale_to_utf8 (buffer, -1, NULL, NULL, NULL);
 #else
         if (!GetLocaleInfoW (GetThreadLocale (), LOCALE_SMONTHNAME1 + i,