diff --git a/include/my_time.h b/include/my_time.h
index d0f2fc323d8..cb8684efdb1 100644
--- a/include/my_time.h
+++ b/include/my_time.h
@@ -97,15 +97,25 @@ int my_date_to_str(const MYSQL_TIME *l_time, char *to);
 int my_datetime_to_str(const MYSQL_TIME *l_time, char *to);
 int my_TIME_to_str(const MYSQL_TIME *l_time, char *to);
 
-/*
-  The following must be sorted so that simple intervals comes first.
-  (get_interval_value() depends on this)
+/* 
+  Available interval types used in any statement.
+
+  'interval_type' must be sorted so that simple intervals comes first,
+  ie year, quarter, month, week, day, hour, etc. The order based on
+  interval size is also important and the intervals should be kept in a
+  large to smaller order. (get_interval_value() depends on this)
+ 
+  Note: If you change the order of elements in this enum you should fix 
+  order of elements in 'interval_type_to_name' and 'interval_names' 
+  arrays 
+  
+  See also interval_type_to_name, get_interval_value, interval_names
 */
 
 enum interval_type
 {
-  INTERVAL_YEAR, INTERVAL_QUARTER, INTERVAL_MONTH, INTERVAL_DAY, INTERVAL_HOUR,
-  INTERVAL_MINUTE, INTERVAL_WEEK, INTERVAL_SECOND, INTERVAL_MICROSECOND ,
+  INTERVAL_YEAR, INTERVAL_QUARTER, INTERVAL_MONTH, INTERVAL_WEEK, INTERVAL_DAY,
+  INTERVAL_HOUR, INTERVAL_MINUTE, INTERVAL_SECOND, INTERVAL_MICROSECOND,
   INTERVAL_YEAR_MONTH, INTERVAL_DAY_HOUR, INTERVAL_DAY_MINUTE,
   INTERVAL_DAY_SECOND, INTERVAL_HOUR_MINUTE, INTERVAL_HOUR_SECOND,
   INTERVAL_MINUTE_SECOND, INTERVAL_DAY_MICROSECOND, INTERVAL_HOUR_MICROSECOND,
diff --git a/mysql-test/r/func_date_add.result b/mysql-test/r/func_date_add.result
index 841d13a6ea6..ac5709260fd 100644
--- a/mysql-test/r/func_date_add.result
+++ b/mysql-test/r/func_date_add.result
@@ -71,3 +71,17 @@ NULL
 NULL
 NULL
 drop table t1;
+End of 4.1 tests
+SELECT CAST('2006-09-26' AS DATE) + INTERVAL 1 DAY;
+CAST('2006-09-26' AS DATE) + INTERVAL 1 DAY
+2006-09-27
+SELECT CAST('2006-09-26' AS DATE) + INTERVAL 1 MONTH;
+CAST('2006-09-26' AS DATE) + INTERVAL 1 MONTH
+2006-10-26
+SELECT CAST('2006-09-26' AS DATE) + INTERVAL 1 YEAR;
+CAST('2006-09-26' AS DATE) + INTERVAL 1 YEAR
+2007-09-26
+SELECT CAST('2006-09-26' AS DATE) + INTERVAL 1 WEEK;
+CAST('2006-09-26' AS DATE) + INTERVAL 1 WEEK
+2006-10-03
+End of 5.0 tests
diff --git a/mysql-test/r/func_time.result b/mysql-test/r/func_time.result
index 82c361edd39..93158750b89 100644
--- a/mysql-test/r/func_time.result
+++ b/mysql-test/r/func_time.result
@@ -612,7 +612,7 @@ date_add(date,INTERVAL "1 1:1:1" DAY_SECOND)
 2003-01-03 01:01:01
 select date_add(date,INTERVAL "1" WEEK) from t1;
 date_add(date,INTERVAL "1" WEEK)
-2003-01-09 00:00:00
+2003-01-09
 select date_add(date,INTERVAL "1" QUARTER) from t1;
 date_add(date,INTERVAL "1" QUARTER)
 2003-04-02
@@ -621,7 +621,7 @@ timestampadd(MINUTE, 1, date)
 2003-01-02 00:01:00
 select timestampadd(WEEK, 1, date) from t1;
 timestampadd(WEEK, 1, date)
-2003-01-09 00:00:00
+2003-01-09
 select timestampadd(SQL_TSI_SECOND, 1, date) from t1;
 timestampadd(SQL_TSI_SECOND, 1, date)
 2003-01-02 00:00:01
diff --git a/mysql-test/t/func_date_add.test b/mysql-test/t/func_date_add.test
index e01fce30577..b575eeececa 100644
--- a/mysql-test/t/func_date_add.test
+++ b/mysql-test/t/func_date_add.test
@@ -64,4 +64,17 @@ insert into t1 values (date_add('2000-01-04', INTERVAL NULL DAY));
 select * from t1;
 drop table t1;
 
-# End of 4.1 tests
+--echo End of 4.1 tests
+
+#
+# Bug#21811
+#
+# Make sure we end up with an appropriate
+# date format (DATE) after addition operation
+#
+SELECT CAST('2006-09-26' AS DATE) + INTERVAL 1 DAY;
+SELECT CAST('2006-09-26' AS DATE) + INTERVAL 1 MONTH;
+SELECT CAST('2006-09-26' AS DATE) + INTERVAL 1 YEAR;
+SELECT CAST('2006-09-26' AS DATE) + INTERVAL 1 WEEK;
+
+--echo End of 5.0 tests
diff --git a/sql/item_timefunc.cc b/sql/item_timefunc.cc
index d32adde5e64..064db11b65f 100644
--- a/sql/item_timefunc.cc
+++ b/sql/item_timefunc.cc
@@ -2030,11 +2030,15 @@ bool Item_date_add_interval::eq(const Item *item, bool binary_cmp) const
           (date_sub_interval == other->date_sub_interval));
 }
 
+/*
+   'interval_names' reflects the order of the enumeration interval_type.
+   See item_timefunc.h
+ */
 
 static const char *interval_names[]=
 {
-  "year", "quarter", "month", "day", "hour",
-  "minute", "week", "second", "microsecond",
+  "year", "quarter", "month", "week", "day",  
+  "hour", "minute", "second", "microsecond",
   "year_month", "day_hour", "day_minute", 
   "day_second", "hour_minute", "hour_second",
   "minute_second", "day_microsecond",
diff --git a/sql/time.cc b/sql/time.cc
index 0461f7723c6..467dbc684a5 100644
--- a/sql/time.cc
+++ b/sql/time.cc
@@ -25,14 +25,25 @@
 
 #ifndef TESTTIME
 
+/*
+  Name description of interval names used in statements.
+
+  'interval_type_to_name' is ordered and sorted on interval size and
+  interval complexity.
+  Order of elements in 'interval_type_to_name' should correspond to 
+  the order of elements in 'interval_type' enum
+  
+  See also interval_type, interval_names
+*/
+
 LEX_STRING interval_type_to_name[INTERVAL_LAST] = {
   { C_STRING_WITH_LEN("YEAR")},
   { C_STRING_WITH_LEN("QUARTER")},
   { C_STRING_WITH_LEN("MONTH")},
+  { C_STRING_WITH_LEN("WEEK")},
   { C_STRING_WITH_LEN("DAY")},
   { C_STRING_WITH_LEN("HOUR")},
   { C_STRING_WITH_LEN("MINUTE")},
-  { C_STRING_WITH_LEN("WEEK")},
   { C_STRING_WITH_LEN("SECOND")},
   { C_STRING_WITH_LEN("MICROSECOND")},
   { C_STRING_WITH_LEN("YEAR_MONTH")},