CREATE TABLE WebRequest (
	ID INT IDENTITY(1,1) PRIMARY KEY,
	VisitDate DATETIME )
CREATE NONCLUSTERED INDEX IX_WebVisitDate ON WebRequest(VisitDate);

DECLARE @TestDate DATETIME = '2009-06-01'
DECLARE @WrongDate DATETIME = '1753-01-01'
INSERT INTO WebRequest (VisitDate)
SELECT @TestDate UNION ALL
SELECT DATEADD(ss, 3, @TestDate) UNION ALL
SELECT DATEADD(ss, 6, @TestDate) UNION ALL
SELECT DATEADD(ss, 9, @TestDate) UNION ALL
SELECT DATEADD(ss, 10, @TestDate) UNION ALL
SELECT @WrongDate UNION ALL
SELECT DATEADD(mi, 3, @TestDate) UNION ALL
SELECT DATEADD(mi, 4, @TestDate) UNION ALL
SELECT DATEADD(mi, 5, @TestDate) UNION ALL
SELECT @WrongDate UNION ALL
SELECT @WrongDate UNION ALL
SELECT @WrongDate UNION ALL
SELECT DATEADD(mi,10, @TestDate) UNION ALL
SELECT DATEADD(mi, 11, @TestDate) UNION ALL
SELECT DATEADD(mi, 15, @TestDate) UNION ALL
SELECT @WrongDate UNION ALL
SELECT @WrongDate UNION ALL
SELECT DATEADD(mi,16, @TestDate) UNION ALL
SELECT DATEADD(mi, 17, @TestDate) UNION ALL
SELECT DATEADD(mi, 18, @TestDate);

GO

BEGIN TRAN VisitTran;

DECLARE @CutOffDate DATETIME = '2009-01-01';
SELECT * FROM WebRequest ORDER BY ID;
; WITH RecursiveCTE(ID, Updated, NewDate) AS (

	SELECT W1.ID, W1.VisitDate, DATEADD(ms, 300, W2.VisitDate)  AS NewDate
	FROM dbo.WebRequest AS W1
	JOIN dbo.WebRequest AS W2 ON W2.ID = W1.ID -1
	WHERE W1.VisitDate < @CutOffDate 
	AND W2.VisitDate  > @CutOffDate
	
	UNION ALL 
	
	SELECT W3.ID, W3.VisitDate, DATEADD(ms, 300, C.NewDate) AS NewDate
	FROM dbo.WebRequest AS W3
	JOIN RecursiveCTE AS C ON C.ID = W3.ID -1
	WHERE W3.VisitDate < @CutOffDate
)
UPDATE WR
	SET WR.VisitDate = C.NewDate
FROM WebRequest AS WR
JOIN RecursiveCTE AS C ON C.ID = WR.ID 
-- OPTION (MAXRECURSION 32767) 

SELECT * FROM WebRequest ORDER BY ID;

ROLLBACK TRAN VisitTran;

DECLARE @CutOffDate DATETIME = '2009-01-01';
;WITH CountPrevWrongDates (ID, VisitDate) AS 
(
	SELECT W1.ID, W1.VisitDate
	FROM dbo.WebRequest AS W1
	JOIN dbo.WebRequest AS W2 ON W2.ID = W1.ID - 1
	WHERE W1.VisitDate < @CutOffDate 
	AND W2.VisitDate < @CutOffDate
	
	UNION ALL
	
	SELECT W3.ID, C.VisitDate
	FROM dbo.WebRequest as W3
	JOIN CountPrevWrongDates AS C ON W3.ID = C.ID -1
	WHERE W3.VisitDate < @CutOffDate
)
SELECT  C1.ID,	COUNT(C1.ID) AS RunLength
FROM CountPrevWrongDates AS C1
JOIN CountPrevWrongDates AS C2 ON C1.ID = C2.ID -1
GROUP BY C1.ID
OPTION (MAXRECURSION 32767);

GO

DECLARE @CutOffDate DATETIME = '2009-01-01';
WITH CTE AS (
SELECT ID, VisitDate,
       ROW_NUMBER() OVER(ORDER BY ID) -
       ROW_NUMBER() OVER(PARTITION BY CASE WHEN VisitDate < @CutOffDate THEN 1 ELSE 0 END ORDER BY ID) AS rnDiff
FROM WebRequest)
SELECT MIN(ID) AS StartID,
       COUNT(*) AS RunLength
FROM CTE
WHERE VisitDate < @CutOffDate
GROUP BY rnDiff
ORDER BY StartID;