DECLARE @Contents varchar(4000)
SET @Contents =
'abcdef 01/01/2018 08:15:13 This is a note , this is a note , this is a note , this is a note , this is a note , this is a note , this is a note, ghijkl 02/02/2018 12:15:45, this is a note , this is a note , this is a note , this is a note mnopqr 03/02/2018 10:34:23 this is a note this is a note this is a note this is a note this is a note this is a note';

WITH Ten AS (
    SELECT n FROM (VALUES (1), (2), (3), (4), (5), (6), (7), (8), (9), (0)) v(n)
    )
, Hundred AS (
    SELECT t1.n
    FROM Ten t1
    CROSS JOIN Ten t2
    )
, Thousand(n) AS (
    SELECT ROW_NUMBER() OVER (ORDER BY (SELECT NULL))
    FROM Hundred h
    CROSS JOIN Ten t
    )
, StartPoints AS (
    SELECT n AS StartChar
    FROM Thousand
    WHERE SUBSTRING(@Contents,n,7) LIKE '[a-z][a-z][a-z][a-z][a-z][a-z] '
    AND ISDATE(SUBSTRING(@Contents,n+7,19)) = 1
    AND n<= LEN(@Contents) - 26
    )
, StartPointssandNext AS (
    SELECT
         StartChar
    ,    LEAD(StartChar,1,LEN(@Contents)+1) OVER (ORDER BY StartChar) AS NextStartChar
    FROM StartPoints
    )
SELECT
    SUBSTRING(@Contents,StartChar,NextStartChar-StartChar)
FROM StartPointssandNext

John