DECLARE @Varmonth CHAR(2)

SELECT @Varmonth = 04

select @Varmonth =datediff(mm,'1899/04/01',DATEADD(mm,CAST(@Varmonth AS INT) ,0))%12 +CASE WHEN CAST(@Varmonth AS INT) = 3 THEN 12 ELSE 0 END

select @Varmonth = REPLICATE('0',2-LEN(@Varmonth))+@Varmonth

select @Varmonth