ADO/ADO.NET - problem in order by using in select unoin query

hi..i have problem in my select with union query in below select statement..

getting invalid column name dt error

Msg 207, Level 16, State 1, Line 2
Invalid column name 'dd'.
Msg 104, Level 16, State 1, Line 2
ORDER BY items must appear in the select list if the statement contains a UNION, INTERSECT or EXCEPT operator.

select convert(datetime,date,103)as dd  from tbl_Newappraisal
union select convert(datetime,issuedate,103)as dd from silk_issue order by convert(datetime,dd,103) asc

Try using this 
select convert(datetime,date,103) 
in your order by statement, not the "dd" alias.