To Calculate Time Difference As HH:MM:SS With Today Date And Column Date

SELECT
 CAST(DATEDIFF(second, Dttm, getdate()) / 60 / 60 / 24 AS NVARCHAR(50)) + ':'+
     CAST(DATEDIFF(second, Dttm, getdate()) / 60 / 60 % 24  AS NVARCHAR(50)) + ':' +
     CAST(DATEDIFF(second, Dttm, getdate()) / 60 % 60 AS NVARCHAR(50)) as Hour
FROM YourTable