SQL query to find the difference between two periods in hh:mm:ss format

DECLARE @StartTime DateTime = Getdate() -1, @EndTime DateTime = GetDate()

SELECTCONVERT(VARCHAR(6),DATEDIFF(SECOND, @STARTTIME, ISNULL(@ENDTIME,GETDATE()))/ 3600)

+':'+ RIGHT('0' + CONVERT(VARCHAR(6), (DATEDIFF(second, @STARTTIME, ISNULL(@ENDTIME,GETDATE())) % 3660) / 60), 2)

+':'+ RIGHT('0' + CONVERT(VARCHAR(5), DATEDIFF(second, @STARTTIME, ISNULL(@ENDTIME,GETDATE())) % 60),2) AS DURATION