You want fries and a coke with that
SELECT t2.U_Username
FROM w3t_Posts AS t1
LEFT JOIN w3t_Users AS t2 ON t1.B_PosterId = t2.U_Number
WHERE B_Posted > UNIX_TIMESTAMP( DATE_SUB( CURDATE( ) , INTERVAL 1
MONTH ) )
GROUP BY t2.U_Username ASC
Change the interval to whatever you want
3 DAY
7 DAY
15 DAY
1 WEEK
3 WEEK
6 WEEK
2 MONTH
6 MONTH
etc etc