;WITH SampleData AS (
	SELECT ID, InsertDateDT = CAST(InsertDate AS DATETIME)
	FROM (VALUES (1,'20120828'), (2,'20120828'), (3,'20120829'),(4,'20120829')) d (ID, InsertDate)
)
SELECT 
	ID,
	InsertDateDT, 
	MyBigintCompositeID = CAST(
		CONVERT(VARCHAR(8),InsertDateDT,112) 
		+ RIGHT('00000'+CAST(ROW_NUMBER() OVER(PARTITION BY InsertDateDT ORDER BY ID) AS VARCHAR(5)),5) 
		AS BIGINT)
FROM SampleData

--------------------------------------------------------------
drop table #test;
create table #test ( id varchar(13) );
--------------------------------------------------------------
declare @id varchar(13), @nextid varchar(13);

select @id = max(id) from #test;

select @nextid = case when left(@id, 8) = convert(varchar(8), getdate(), 112)
						then convert(varchar(8), getdate(), 112) + 
							right('0000' + cast(cast(substring(@id, 9, 5) as int) +1 as varchar(5)),5)
						else convert(varchar(8), getdate(), 112) + '00001'
					end;

insert into #test values ( @nextid );
--------------------------------------------------------------
select * from #test;
--------------------------------------------------------------

CREATE TABLE #t (ID BIGINT IDENTITY(201208280001, 1), value INT)

INSERT INTO #t SELECT 1 UNION ALL SELECT 2 UNION ALL SELECT 3

IF LEFT(IDENT_CURRENT('#t'), 8) <> CONVERT(VARCHAR(8), GETDATE(), 112)
BEGIN
    DECLARE @newseed BIGINT = CAST(CONVERT(VARCHAR(8), GETDATE(), 112)+'0000' AS BIGINT)
    DBCC CHECKIDENT('#t', RESEED, @newseed)
END

INSERT INTO #t SELECT 4 UNION ALL SELECT 5 UNION ALL SELECT 6

SELECT * FROM #t

DROP TABLE #t