how to convert date string in vb.net or SQL 2005

Try the below:

declare @s varchar(50)='27/12/2014'

;With cte as(Select parsename(REPLACE(@s,'/','.'),3) sday, parsename(REPLACE(@s,'/','.'),2) sMon
,Left(parsename(REPLACE(@s,'/','.'),1),2) sYear1,Right(parsename(REPLACE(@s,'/','.'),1),2) sYear2 )

Select RIGHT(sday,1)+Left(sday,1)+CHAR(sMon-1+ASCII('A'))
                +RIGHT(sYear1,1)+ LEFT(sYEar1,1)+':'+RIGHT(sYear2,1)+ LEFT(sYEar2,1) From cte