Ummm...I think you are over-thinking this.
If you include a TIMESTAMP or DATETIME field in your record that records the time the posting was made, it's trivial. Just exclude older postings when listing or searching the postings.
Example: If you had a field
whenPosted TIMESTAMP DEFAULT CURRENTTIMESTAMP column in your table, then you would just include that in all appropriate SELECTs:
Code:
SELECT * FROM postings WHERE whenPosted > DATE_SUB( CURDATE(), INTERVAL 5 DAY )
You *could* even use a VIEW to make this easier:
Code:
CREATE VIEW searchableLPostings
AS
SELECT * FROM postings WHERE whenPosted > DATE_SUB( CURDATE(), INTERVAL 5 DAY )
And now for all searches, etc., you use
Code:
SELECT ... FROM searchablePostings WHERE ...
and your searches won't even see any older posts.