CREATE TABLE #MyDates
    (SLN INT, ID INT, StartDate DATETIME, EndDate DATETIME
    ,Duration AS (DATEDIFF(minute, StartDate, EndDate))
    ,TimeElapsed BIGINT
    ,PRIMARY KEY (SLN, StartDate))

INSERT INTO #MyDates (SLN, ID, StartDate, EndDate)
SELECT 1, 1, '1/1/2012 10:12','1/1/2012 10:13'
UNION ALL SELECT 1, 1, '1/1/2012 10:14','1/1/2012 10:15'
UNION ALL SELECT 1, 1, '1/1/2012 10:15','1/2/2012 10:16'
UNION ALL SELECT 2, 2, '1/1/2012 10:12','1/1/2012 10:13'
UNION ALL SELECT 2, 2, '1/1/2012 10:14','1/1/2012 10:15'
UNION ALL SELECT 2, 2, '1/1/2012 10:15','1/2/2012 10:16'

DECLARE @StartTime DATETIME = '1900-01-01'
    ,@SLN   INT = 0

UPDATE #MyDates WITH(TABLOCKX)
SET TimeElapsed = DATEDIFF(minute, CASE WHEN @SLN = SLN THEN @StartTime ELSE StartDate END, EndDate)
    ,@StartTime = CASE WHEN @SLN <> SLN THEN StartDate ELSE @StartTime END
    ,@SLN = SLN
OPTION (MAXDOP 1)

SELECT SLN, ID, StartDate, EndDate
    ,Duration=CAST(Duration / 1440 AS VARCHAR(3)) + ' day' + 
        CASE Duration / 1440 WHEN 1 THEN ' ' ELSE 's ' END +
        LEFT(DATEADD(minute, Duration % 1440, CAST('00:00' AS TIME)), 8)
    ,ElapsedTime=CAST(TimeElapsed / 1440 AS VARCHAR(3)) + ' day' + 
        CASE TimeElapsed / 1440 WHEN 1 THEN ' ' ELSE 's ' END +
        LEFT(DATEADD(minute, TimeElapsed % 1440, CAST('00:00' AS TIME)), 8)
FROM #MyDates

DROP TABLE #MyDates