diff --git a/src/backend/utils/adt/formatting.c b/src/backend/utils/adt/formatting.c
index fcfe6ed4ba8..54213afb05a 100644
--- a/src/backend/utils/adt/formatting.c
+++ b/src/backend/utils/adt/formatting.c
@@ -114,6 +114,7 @@
 #define DCH_MAX_ITEM_SIZ	   12	/* max localized day name		*/
 #define NUM_MAX_ITEM_SIZ		8	/* roman number (RN has 15 chars)	*/
 
+#define MAX_L10N_DATA			80	/* max localized day or month name */
 
 /* ----------
  * Format parser structs
@@ -1062,8 +1063,8 @@ static int	from_char_parse_int_len(int *dest, const char **src, const int len,
 									FormatNode *node, Node *escontext);
 static int	from_char_parse_int(int *dest, const char **src, FormatNode *node,
 								Node *escontext);
-static int	seq_search_ascii(const char *name, const char *const *array, int *len);
-static int	seq_search_localized(const char *name, char **array, int *len,
+static int	seq_search_ascii(const char *name, const char *const *array, size_t *len);
+static int	seq_search_localized(const char *name, char **array, size_t *len,
 								 Oid collid);
 static bool from_char_seq_search(int *dest, const char **src,
 								 const char *const *array,
@@ -2589,7 +2590,7 @@ from_char_parse_int(int *dest, const char **src, FormatNode *node,
  * suitable for comparisons to ASCII strings.
  */
 static int
-seq_search_ascii(const char *name, const char *const *array, int *len)
+seq_search_ascii(const char *name, const char *const *array, size_t *len)
 {
 	unsigned char firstc;
 	const char *const *a;
@@ -2634,6 +2635,31 @@ seq_search_ascii(const char *name, const char *const *array, int *len)
 	return -1;
 }
 
+/*
+ * Compare 'name' with 'element' in a case-insensitive way, by first
+ * converting 'name' to upper case, then lower case. ('element' is already
+ * case-folded that way.)
+ *
+ * A helper function for seq_search_localized().
+ */
+static bool
+casefold_str_cmp(const char *name, size_t name_len,
+				 const char *element, size_t element_len,
+				 Oid collid)
+{
+	char	   *upper_substr;
+	char	   *lower_substr;
+	bool		result;
+
+	upper_substr = str_toupper(name, name_len, collid);
+	lower_substr = str_tolower(upper_substr, strlen(upper_substr), collid);
+	result = strcmp(lower_substr, element) == 0;
+
+	pfree(lower_substr);
+	pfree(upper_substr);
+	return result;
+}
+
 /*
  * Sequentially search an array of possibly non-English words for
  * a case-insensitive match to the initial character(s) of "name".
@@ -2646,9 +2672,10 @@ seq_search_ascii(const char *name, const char *const *array, int *len)
  * the arrays exported by pg_locale.c aren't const.
  */
 static int
-seq_search_localized(const char *name, char **array, int *len, Oid collid)
+seq_search_localized(const char *name, char **array, size_t *len, Oid collid)
 {
-	char	  **a;
+	size_t		name_len = strlen(name);
+	const char *name_end = name + name_len;
 	char	   *upper_name;
 	char	   *lower_name;
 
@@ -2662,9 +2689,9 @@ seq_search_localized(const char *name, char **array, int *len, Oid collid)
 	 * The case-folding processing done below is fairly expensive, so before
 	 * doing that, make a quick pass to see if there is an exact match.
 	 */
-	for (a = array; *a != NULL; a++)
+	for (char **a = array; *a != NULL; a++)
 	{
-		int			element_len = strlen(*a);
+		size_t		element_len = strlen(*a);
 
 		if (strncmp(name, *a, element_len) == 0)
 		{
@@ -2677,30 +2704,100 @@ seq_search_localized(const char *name, char **array, int *len, Oid collid)
 	 * Fold to upper case, then to lower case, so that we can match reliably
 	 * even in languages in which case conversions are not injective.
 	 */
-	upper_name = str_toupper(unconstify(char *, name), strlen(name), collid);
+	upper_name = str_toupper(name, name_len, collid);
 	lower_name = str_tolower(upper_name, strlen(upper_name), collid);
 	pfree(upper_name);
 
-	for (a = array; *a != NULL; a++)
+	for (char **a = array; *a != NULL; a++)
 	{
 		char	   *upper_element;
+		size_t		upper_element_len;
 		char	   *lower_element;
-		int			element_len;
+		size_t		lower_element_len;
 
 		/* Likewise upper/lower-case array element */
 		upper_element = str_toupper(*a, strlen(*a), collid);
-		lower_element = str_tolower(upper_element, strlen(upper_element),
-									collid);
+		upper_element_len = strlen(upper_element);
+		lower_element = str_tolower(upper_element, upper_element_len, collid);
+		lower_element_len = strlen(lower_element);
 		pfree(upper_element);
-		element_len = strlen(lower_element);
 
-		/* Match? */
-		if (strncmp(lower_name, lower_element, element_len) == 0)
+		/* Is 'lower_element' a prefix of 'lower_name' ? */
+		if (strncmp(lower_name, lower_element, lower_element_len) == 0)
 		{
-			*len = element_len;
-			pfree(lower_element);
-			pfree(lower_name);
-			return a - array;
+			/*
+			 * We have a match, but we still need to figure out how long the
+			 * match is.  The case conversions could have changed the lengths
+			 * of either string, or both.
+			 */
+			const char *ep;
+			const char *element_end;
+			size_t		element_nchars;
+			size_t		substr_len;
+			size_t		substr_nchars;
+
+			/*
+			 * First, check the easy case that the string matches as whole.
+			 */
+			if (strlen(lower_name) == lower_element_len)
+			{
+				*len = name_len;
+				pfree(lower_element);
+				pfree(lower_name);
+				return a - array;
+			}
+
+			/*
+			 * Another good guess is that the case conversions did not change
+			 * the number of characters.
+			 */
+
+			/* count characters in the element */
+			ep = lower_element;
+			element_end = lower_element + lower_element_len;
+			for (element_nchars = 0; ep < element_end; element_nchars++)
+				ep += pg_mblen_range(ep, element_end);
+
+			/*
+			 * count the byte length of a substring of 'name' having the same
+			 * character count as the element
+			 */
+			substr_len = 0;
+			for (substr_nchars = 0;
+				 substr_nchars < element_nchars && substr_len < name_len;
+				 substr_nchars++)
+			{
+				substr_len += pg_mblen_range(name + substr_len, name_end);
+			}
+
+			if (casefold_str_cmp(name, substr_len, lower_element, lower_element_len, collid))
+			{
+				*len = substr_len;
+				pfree(lower_element);
+				pfree(lower_name);
+				return a - array;
+			}
+
+			/*
+			 * As last resort, try the case conversion and comparison for
+			 * every substring from the beginning of the original string until
+			 * we find a match.
+			 */
+			substr_len = 0;
+			while (substr_len < name_len)
+			{
+				substr_len += pg_mblen_range(name + substr_len, name_end);
+
+				if (casefold_str_cmp(name, substr_len,
+									 lower_element, lower_element_len,
+									 collid))
+				{
+					*len = substr_len;
+					pfree(lower_element);
+					pfree(lower_name);
+					return a - array;
+				}
+			}
 		}
 		pfree(lower_element);
 	}
@@ -2734,7 +2831,7 @@ from_char_seq_search(int *dest, const char **src, const char *const *array,
 					 char **localized_array, Oid collid,
 					 FormatNode *node, Node *escontext)
 {
-	int			len;
+	size_t		len;
 
 	if (localized_array == NULL)
 		*dest = seq_search_ascii(*src, array, &len);
diff --git a/src/test/regress/expected/collate.linux.utf8.out b/src/test/regress/expected/collate.linux.utf8.out
index 01664f7c1b5..e0dc13881c0 100644
--- a/src/test/regress/expected/collate.linux.utf8.out
+++ b/src/test/regress/expected/collate.linux.utf8.out
@@ -477,6 +477,24 @@ SELECT to_date('01 Şub 2010', 'DD TMMON YYYY');
 SELECT to_date('1234567890ab 2010', 'TMMONTH YYYY'); -- fail
 ERROR:  invalid value "1234567890ab" for "MONTH"
 DETAIL:  The given value did not match any of the allowed values for this field.
+SELECT to_date('01 Aralık 2010', 'DD TMMONTH YYYY');
+  to_date   
+------------
+ 12-01-2010
+(1 row)
+
+SELECT to_date('01 aralık 2010', 'DD TMMONTH YYYY');
+  to_date   
+------------
+ 12-01-2010
+(1 row)
+
+SELECT to_date('2010 01 araLık', 'YYYY DD TMMONTH');
+  to_date   
+------------
+ 12-01-2010
+(1 row)
+
 -- backwards parsing
 CREATE VIEW collview1 AS SELECT * FROM collate_test1 WHERE b COLLATE "C" >= 'bbc';
 CREATE VIEW collview2 AS SELECT a, b FROM collate_test1 ORDER BY b COLLATE "C";
diff --git a/src/test/regress/sql/collate.linux.utf8.sql b/src/test/regress/sql/collate.linux.utf8.sql
index 132d13af0a8..6d726ee9c99 100644
--- a/src/test/regress/sql/collate.linux.utf8.sql
+++ b/src/test/regress/sql/collate.linux.utf8.sql
@@ -188,6 +188,9 @@ SELECT to_date('01 ŞUB 2010', 'DD TMMON YYYY');
 SELECT to_date('01 Şub 2010', 'DD TMMON YYYY');
 SELECT to_date('1234567890ab 2010', 'TMMONTH YYYY'); -- fail
 
+SELECT to_date('01 Aralık 2010', 'DD TMMONTH YYYY');
+SELECT to_date('01 aralık 2010', 'DD TMMONTH YYYY');
+SELECT to_date('2010 01 araLık', 'YYYY DD TMMONTH');
 
 -- backwards parsing