demo, Sql Server - rextester

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/***** DATA EXAMPLE *****/
DECLARE @T TABLE (d1 DATETIME, d2 DATETIME)
INSERT INTO @T (d1, d2)
SELECT '2018-03-26 08:40:00.000','2018-03-26 17:09:00.000' UNION ALL 
SELECT '2018-03-27 09:26:00.000','2018-03-27 13:36:00.000' UNION ALL 
SELECT '2018-03-26 13:00:00.000','2018-03-27 09:26:00.000' UNION ALL 
SELECT '2018-03-26 08:00:00.000','2018-03-26 08:40:00.000';

/*user1045402*/
SELECT 
       s1.d1,
       --t1.d2 
       MIN(t1.d2) AS d2
FROM @T s1 
INNER JOIN @T t1 ON s1.d1 <= t1.d2
  AND NOT EXISTS(SELECT * FROM @T t2 
                 WHERE t1.d2 >= t2.d1 AND t1.d2 < t2.d2) 
WHERE NOT EXISTS(SELECT * FROM @T s2 
                 WHERE s1.d1 > s2.d1 AND s1.d1 <= s2.d2) 
GROUP BY s1.d1 
ORDER BY s1.d1;

/*martin*/
    WITH T1
         AS (SELECT *,
                    MAX(d2) OVER (ORDER BY d1) AS max_d2_so_far
             FROM   @T),
         T2
         AS (SELECT *,
                    CASE
                      WHEN d1 <= LAG(max_d2_so_far) OVER (ORDER BY d1)
                        THEN 0
                      ELSE 1
                    END AS range_start
             FROM   T1),
         T3
         AS (SELECT *,
                    SUM(range_start) OVER (ORDER BY d1) AS range_group
             FROM   T2)
    SELECT range_group,
           MIN(d1) AS d1,
           MAX(d2) AS d2
    FROM   T3
    GROUP  BY range_group

View schema
Execution time: 0,03 sec, rows selected: 2, rows affected: 4, absolute service time: 0,17 sec