Unable to compare text datatype fields to extract salient data

UserName Time FieldValue
Bob Hope 12:00:00 We cannot do anything until 1 O'Clock - the penguin needs defrosting
Rolf Harris 13:00:00 We cannot do anything until 1 O'Clock - the penguin needs defrosting I have defrosted it
Jeff Bridges 13:13:00 We cannot do anything until 1 O'Clock - the penguin needs defrosting I have defrosted it It's Alive!
Jenny Bond 13:46:25 We cannot do anything until 1 O'Clock - the penguin needs defrosting I have defrosted it It's Alive! I let it go.

UserName Time FieldValue
Bob Hope 12:00:00 We cannot do anything until 1 O'Clock - the penguin needs defrosting
Rolf Harris 13:00:00 I have defrosted it
Jeff Bridges 13:13:00 It's Alive!
Jenny Bond 13:46:25 I let it go.

-- sample data
DROP TABLE #Sample
CREATE TABLE #Sample (UserName VARCHAR(15), [Time] CHAR(8), FieldValue TEXT)
INSERT INTO #Sample (UserName, [Time], FieldValue)
SELECT 'Bob Hope', '12:00:00', 'We cannot do anything until 1 O''Clock - the penguin needs defrosting'
UNION ALL SELECT 'Rolf Harris', '13:00:00', 'We cannot do anything until 1 O''Clock - the penguin needs defrosting I have defrosted it'
UNION ALL SELECT 'Jeff Bridges', '13:13:00', 'We cannot do anything until 1 O''Clock - the penguin needs defrosting I have defrosted it It''s Alive!'
UNION ALL SELECT 'Jenny Bond', '13:46:25', 'We cannot do anything until 1 O''Clock - the penguin needs defrosting I have defrosted it It''s Alive! I let it go.'

-- solution
;WITH SequencedData AS (
SELECT
UserName,
[Time],
FieldValue = CAST(FieldValue AS VARCHAR(MAX)),
rn = ROW_NUMBER() OVER (ORDER BY [Time])
FROM #Sample
),
rCTE AS (
SELECT UserName, [Time], rn,
UserString = FieldValue,
FieldValue = FieldValue
FROM SequencedData
WHERE rn = 1
UNION ALL
SELECT tr.UserName, tr.[Time], tr.rn,
UserString = LTRIM(REPLACE(tr.FieldValue,lr.FieldValue,'')),
FieldValue = tr.FieldValue
FROM SequencedData tr
INNER JOIN rCTE lr ON lr.rn+1 = tr.rn
)
SELECT UserName, [Time], UserString
FROM rCTE

IF OBJECT_ID(N'tempdb..#T') IS NOT NULL
DROP TABLE #T;

CREATE TABLE #T
(EntryID INT NOT NULL,
EntryTime DATETIME NOT NULL,
FieldValue VARCHAR(MAX));

INSERT INTO #T
(EntryID, EntryTime, FieldValue)
VALUES (1, '20120924 00:00:00', 'We cannot do anything until 1 O''Clock - the penguin needs defrosting'),
(1, '20120924 13:00:00',
'We cannot do anything until 1 O''Clock - the penguin needs defrosting I have defrosted it'),
(1, '20120924 13:05:00',
'We cannot do anything until 1 O''Clock - the penguin needs defrosting I have defrosted it It''s Alive!'),
(1, '20120924 13:10:00',
'We cannot do anything until 1 O''Clock - the penguin needs defrosting I have defrosted it It''s Alive! I let it go.');

SELECT T1.EntryTime,
LTRIM(STUFF(T1.FieldValue, 1, ISNULL(LEN(Previous.FieldValue), 0), ''))
FROM #T AS T1
OUTER APPLY (SELECT TOP (1)
*
FROM #T AS T2
WHERE T1.EntryID = T2.EntryID
AND T1.EntryTime > T2.EntryTime
ORDER BY EntryTime DESC) AS Previous
ORDER BY T1.EntryTime;

create table #SomeData
(
UserName varchar(20),
EntryTime Time,
FieldValue varchar(max)
)

insert #SomeData
select 'Bob Hope', '12:00:00', 'We cannot do anything until 1 O''Clock - the penguin needs defrosting' union all
select 'Rolf Harris', '13:00:00', 'We cannot do anything until 1 O''Clock - the penguin needs defrosting I have defrosted it' union all
select 'Jeff Bridges', '13:13:00', 'We cannot do anything until 1 O''Clock - the penguin needs defrosting I have defrosted it It''s Alive!' union all
select 'Jenny Bond', '13:46:25', 'We cannot do anything until 1 O''Clock - the penguin needs defrosting I have defrosted it It''s Alive! I let it go.'

;with cte as
(
select *, ROW_NUMBER()over(order by EntryTime) as RowNum
from #SomeData
)
, cte2 as
(
SELECT UserName, EntryTime, c1.FieldValue as FieldValue1, c1.RowNum, (select c2.FieldValue from cte c2 where c2.RowNum = c1.RowNum - 1) as FieldValue2 FROM cte c1
)

select UserName, EntryTime, ltrim(isnull(REPLACE(FieldValue1, FieldValue2, ''), FieldValue1)) as NewValue from cte2

drop table #SomeData

GSquared (9/24/2012)

Another possible:

IF OBJECT_ID(N'tempdb..#T') IS NOT NULL 
    DROP TABLE #T;
	
CREATE TABLE #T
    (EntryID INT NOT NULL,
     EntryTime DATETIME NOT NULL,
     FieldValue VARCHAR(MAX));

INSERT  INTO #T
        (EntryID, EntryTime, FieldValue)
VALUES  (1, '20120924 00:00:00', 'We cannot do anything until 1 O''Clock - the penguin needs defrosting'),
        (1, '20120924 13:00:00',
         'We cannot do anything until 1 O''Clock - the penguin needs defrosting  I have defrosted it'),
        (1, '20120924 13:05:00',
         'We cannot do anything until 1 O''Clock - the penguin needs defrosting  I have defrosted it  It''s Alive!'),
        (1, '20120924 13:10:00',
         'We cannot do anything until 1 O''Clock - the penguin needs defrosting  I have defrosted it  It''s Alive!  I let it go.');

SELECT  T1.EntryTime,
        LTRIM(STUFF(T1.FieldValue, 1, ISNULL(LEN(Previous.FieldValue), 0), ''))
FROM    #T AS T1
        OUTER APPLY (SELECT TOP (1)
                            *
                     FROM   #T AS T2
                     WHERE  T1.EntryID = T2.EntryID
                            AND T1.EntryTime > T2.EntryTime
                     ORDER BY EntryTime DESC) AS Previous
ORDER BY T1.EntryTime;

Outer Apply grabs the immediately prior row, if any, gets the Len() of the value, and uses Stuff() to get rid of that much of the beginning of the string.

Using Replace() (as per Chris idea), might not work if a string can repeat itself inside the data. If, for example, the first entry were "Hello!", and the second were then "Hello! Hello! to you too", you'd end up with the "Hello!" substring stripped out where it's not supposed to be. If that's not a possible scenario (repeating string), then it won't matter and either will work.

-- sample data
DROP TABLE #Sample
CREATE TABLE #Sample (UserName VARCHAR(15), [Time] CHAR(8), FieldValue TEXT)
INSERT INTO #Sample (UserName, [Time], FieldValue)
SELECT 'Bob Hope', '12:00:00', 'We cannot do anything until 1 O''Clock - the penguin needs defrosting'
UNION ALL SELECT 'Rolf Harris', '13:00:00', 'We cannot do anything until 1 O''Clock - the penguin needs defrosting I have defrosted it'
UNION ALL SELECT 'Jeff Bridges', '13:13:00', 'We cannot do anything until 1 O''Clock - the penguin needs defrosting I have defrosted it It''s Alive!'
UNION ALL SELECT 'Jenny Bond', '13:46:25', 'We cannot do anything until 1 O''Clock - the penguin needs defrosting I have defrosted it It''s Alive! I let it go.'

-- solution
;WITH SequencedData AS (
SELECT
UserName,
[Time],
FieldValue = CAST(FieldValue AS VARCHAR(MAX)),
rn = ROW_NUMBER() OVER (ORDER BY [Time])
FROM #Sample
),
rCTE AS (
SELECT UserName, [Time], rn,
UserString = FieldValue,
FieldValue = FieldValue
FROM SequencedData
WHERE rn = 1
UNION ALL
SELECT tr.UserName, tr.[Time], tr.rn,
UserString = LTRIM(STUFF(tr.FieldValue,1,DATALENGTH(lr.FieldValue),'')),
FieldValue = tr.FieldValue
FROM SequencedData tr
INNER JOIN rCTE lr ON lr.rn+1 = tr.rn
)
SELECT UserName, [Time], UserString
FROM rCTE