SQL Function Get Total Time in HH:MM:SS

CREATE FUNCTION [dbo].TotalTime
(
  @StartTime DATETIME,
  @EndTime DATETIME
)
RETURNS Varchar(10)
AS
begin
DECLARE @D VARCHAR(400)='31784'
Declare @outTime Varchar(10);
SELECT @D = DATEDIFF(SECOND,@StartTime,@EndTime)


 SELECT @outTime =CONVERT(VARCHAR(5), @D/60/60)
  + ':' + RIGHT('0' + CONVERT(VARCHAR(2), @D/60%60), 2)
  + ':' + RIGHT('0' + CONVERT(VARCHAR(2), @D % 60), 2)


 RETURN  @outTime
END
GO




   SELECT dbo.TotalTime('2016-08-10 10:36:01.000','2016-08-10 19:25:45.000')