I don't have access to an Oracle database. I did this solution using SQL Server 2012. Maybe you can use it to help you with your Oracle solution.
The requirement that a "day" for your query is from 6:00AM on one day until 6:00AM on the next day presents a bit of a challenge. To solve that, I first used a Common Table Expression to sum the values by a date. I use the CASE statement to compute the date. If the time is less than 6AM, then the date is set to the day before. After creating the CTE, I select the results and format the "Date" column to your specification including the dashes in the date.
Create the table
SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO
CREATE TABLE [dbo].[tblLog](
[TimeStamp] [datetime] not NULL,
[Value] [decimal](18, 3) not NULL
) ON [PRIMARY]
GO
Put some data into the table
insert into tblLog values('2014-09-01 07:00:00',100.1);
insert into tblLog values('2014-09-01 08:00:00',100.1);
insert into tblLog values('2014-09-02 07:00:00',100.1);
insert into tblLog values('2014-09-02 08:00:00',100.1);
insert into tblLog values('2014-09-03 07:00:00',100.1);
insert into tblLog values('2014-09-03 08:00:00',100.1);
insert into tblLog values('2014-09-01 05:00:00',100.1);
insert into tblLog values('2014-09-01 06:00:00',100.1);
insert into tblLog values('2014-09-02 05:00:00',100.1);
insert into tblLog values('2014-09-02 06:00:00',100.1);
insert into tblLog values('2014-09-03 05:00:00',100.1);
insert into tblLog values('2014-09-03 06:00:00',100.1);
Execute the query
With Sums (LogDate,Sum)
AS
(
SELECT
CASE
WHEN CAST(timestamp AS time)<'06:00 am' THEN DATEADD(day,-1,CAST(timestamp AS date)) ELSE CAST(timestamp AS date)
END AS LogDate,
SUM(value) AS Sum
FROM tblLog
GROUP BY
CASE
WHEN CAST(timestamp AS time)<'06:00 am' THEN DATEADD(day,-1,CAST(timestamp AS date)) ELSE CAST(timestamp AS date)
END
)
SELECT CONCAT(REPLACE(CONVERT(NVARCHAR,LogDate,113),' ','-'),' 06 AM - ',REPLACE(CONVERT(NVARCHAR,DATEADD(day,1,LogDate),113),' ','-'),' 06 AM') AS LogDate,Sum from Sums
ORDER BY CAST(LogDate AS DATE)
The results
LogDate Sum
31-Aug-2014 06 AM - 01-Sep-2014 06 AM 100.100
01-Sep-2014 06 AM - 02-Sep-2014 06 AM 400.400
02-Sep-2014 06 AM - 03-Sep-2014 06 AM 400.400
03-Sep-2014 06 AM - 04-Sep-2014 06 AM 300.300