SQL Server 2014

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CREATE TABLE myDtTm (theDateTime datetime);

INSERT INTO myDtTm (theDateTime)
VALUES ('2012-08-28 12:05 PM')
      ,('2012-08-28 12:08 PM')
      ,('2012-08-28 12:11 PM')
      ,('2012-08-28 12:15 PM')
      ,('2012-08-28 12:20 PM')
      ,('2012-08-28 12:24 PM')
      ,('2012-08-28 12:28 PM')
      ,('2012-08-28 12:32 PM')
      ,('2012-08-28 12:36 PM')
      ,('2012-08-28 12:39 PM')
      ,('2012-08-28 12:42 PM')
      ,('2012-08-28 12:45 PM')
      ,('2012-08-28 4:01 PM')
      ,('2012-08-28 4:05 PM')
      ,('2012-08-28 4:08 PM')
      ,('2012-08-28 4:11 PM')
      ,('2012-08-28 4:14 PM')
      ,('2012-08-28 4:20 PM')
      ,('2012-08-28 4:23 PM')
      ,('2012-08-28 4:28 PM')
      ,('2012-08-28 4:33 PM')
      ,('2012-08-28 4:36 PM')
      ,('2012-08-28 4:40 PM')
      ,('2012-08-28 4:44 PM')
      ,('2012-08-28 4:47 PM')
      ,('2012-08-28 8:01 PM')
;
26 rows affected

SELECT theDateTime
      ,SUM(TimeDiff) OVER (ORDER BY theDateTime)
  FROM (
        SELECT
               theDateTime
              ,CASE WHEN COALESCE(DATEDIFF(minute, LAG(theDateTime, 1) OVER (ORDER BY theDateTime), theDateTime), 100000) > 10
                 THEN 1
                 ELSE 0
               END as TimeDiff
          FROM myDtTm
       ) sq
 ORDER BY theDateTime
;


theDateTime
(No column name)


28/08/2012 12:05:00
1

28/08/2012 12:08:00
1

28/08/2012 12:11:00
1

28/08/2012 12:15:00
1

28/08/2012 12:20:00
1

28/08/2012 12:24:00
1

28/08/2012 12:28:00
1

28/08/2012 12:32:00
1

28/08/2012 12:36:00
1

28/08/2012 12:39:00
1

28/08/2012 12:42:00
1

28/08/2012 12:45:00
1

28/08/2012 16:01:00
2

28/08/2012 16:05:00
2

28/08/2012 16:08:00
2

28/08/2012 16:11:00
2

28/08/2012 16:14:00
2

28/08/2012 16:20:00
2

28/08/2012 16:23:00
2

28/08/2012 16:28:00
2

28/08/2012 16:33:00
2

28/08/2012 16:36:00
2

28/08/2012 16:40:00
2

28/08/2012 16:44:00
2

28/08/2012 16:47:00
2

28/08/2012 20:01:00
3

theDateTime	(No column name)
28/08/2012 12:05:00	1
28/08/2012 12:08:00	1
28/08/2012 12:11:00	1
28/08/2012 12:15:00	1
28/08/2012 12:20:00	1
28/08/2012 12:24:00	1
28/08/2012 12:28:00	1
28/08/2012 12:32:00	1
28/08/2012 12:36:00	1
28/08/2012 12:39:00	1
28/08/2012 12:42:00	1
28/08/2012 12:45:00	1
28/08/2012 16:01:00	2
28/08/2012 16:05:00	2
28/08/2012 16:08:00	2
28/08/2012 16:11:00	2
28/08/2012 16:14:00	2
28/08/2012 16:20:00	2
28/08/2012 16:23:00	2
28/08/2012 16:28:00	2
28/08/2012 16:33:00	2
28/08/2012 16:36:00	2
28/08/2012 16:40:00	2
28/08/2012 16:44:00	2
28/08/2012 16:47:00	2
28/08/2012 20:01:00	3