diff --git a/mysql-test/r/func_time.result b/mysql-test/r/func_time.result
index d8ba606a558..47a0f83802c 100644
--- a/mysql-test/r/func_time.result
+++ b/mysql-test/r/func_time.result
@@ -695,3 +695,24 @@ t1	CREATE TABLE `t1` (
   `from_unixtime(1) + 0` double(23,6) default NULL
 ) ENGINE=MyISAM DEFAULT CHARSET=latin1
 drop table t1;
+(select time_format(timediff(now(), DATE_SUB(now(),INTERVAL 5 DAY)),'%H') As H)
+union
+(select time_format(timediff(now(), DATE_SUB(now(),INTERVAL 5 DAY)),'%H') As H);
+H
+120
+(select time_format(timediff(now(), DATE_SUB(now(),INTERVAL 5 DAY)),'%k') As H)
+union
+(select time_format(timediff(now(), DATE_SUB(now(),INTERVAL 5 DAY)),'%k') As H);
+H
+120
+(select time_format(timediff(now(), DATE_SUB(now(),INTERVAL 5 HOUR)),'%H') As H)
+union
+(select time_format(timediff(now(), DATE_SUB(now(),INTERVAL 5 HOUR)),'%H') As H);
+H
+05
+(select time_format(timediff(now(), DATE_SUB(now(),INTERVAL 5 HOUR)),'%k') As H)
+union
+(select time_format(timediff(now(), DATE_SUB(now(),INTERVAL 5 HOUR)),'%k') As H);
+H
+5
+End of 4.1 tests
diff --git a/mysql-test/t/func_time.test b/mysql-test/t/func_time.test
index b8647a281d4..472f3d81d2b 100644
--- a/mysql-test/t/func_time.test
+++ b/mysql-test/t/func_time.test
@@ -368,4 +368,22 @@ create table t1 select now() - now(), curtime() - curtime(),
 show create table t1;
 drop table t1;
 
-# End of 4.1 tests
+#
+# Bug #19844 time_format in Union truncates values
+#
+
+(select time_format(timediff(now(), DATE_SUB(now(),INTERVAL 5 DAY)),'%H') As H)
+union
+(select time_format(timediff(now(), DATE_SUB(now(),INTERVAL 5 DAY)),'%H') As H);
+(select time_format(timediff(now(), DATE_SUB(now(),INTERVAL 5 DAY)),'%k') As H)
+union
+(select time_format(timediff(now(), DATE_SUB(now(),INTERVAL 5 DAY)),'%k') As H);
+(select time_format(timediff(now(), DATE_SUB(now(),INTERVAL 5 HOUR)),'%H') As H)
+union
+(select time_format(timediff(now(), DATE_SUB(now(),INTERVAL 5 HOUR)),'%H') As H);
+
+(select time_format(timediff(now(), DATE_SUB(now(),INTERVAL 5 HOUR)),'%k') As H)
+union
+(select time_format(timediff(now(), DATE_SUB(now(),INTERVAL 5 HOUR)),'%k') As H);
+
+--echo End of 4.1 tests
diff --git a/sql/item_timefunc.cc b/sql/item_timefunc.cc
index 44d9b422263..febc92e34f6 100644
--- a/sql/item_timefunc.cc
+++ b/sql/item_timefunc.cc
@@ -1600,14 +1600,12 @@ uint Item_func_date_format::format_length(const String *format)
       case 'u': /* week (00..52), where week starts with Monday */
       case 'V': /* week 1..53 used with 'x' */
       case 'v': /* week 1..53 used with 'x', where week starts with Monday */
-      case 'H': /* hour (00..23) */
       case 'y': /* year, numeric, 2 digits */
       case 'm': /* month, numeric */
       case 'd': /* day (of the month), numeric */
       case 'h': /* hour (01..12) */
       case 'I': /* --||-- */
       case 'i': /* minutes, numeric */
-      case 'k': /* hour ( 0..23) */
       case 'l': /* hour ( 1..12) */
       case 'p': /* locale's AM or PM */
       case 'S': /* second (00..61) */
@@ -1616,6 +1614,10 @@ uint Item_func_date_format::format_length(const String *format)
       case 'e': /* day (0..31) */
 	size += 2;
 	break;
+      case 'k': /* hour ( 0..23) */
+      case 'H': /* hour (00..23; value > 23 OK, padding always 2-digit) */
+	size += 7; /* docs allow > 23, range depends on sizeof(unsigned int) */
+	break;
       case 'r': /* time, 12-hour (hh:mm:ss [AP]M) */
 	size += 11;
 	break;