How to get records of last 15 days of [Tag_Id=96] ?

Hi, 
The following query is working fine to get last records of [tag_id=96], but now i want to get last 15 days records 
where [tag_id=96]?
How to get records of last 15 days of [Tag_Id=96] ?
I'm using Oracle 11g.
 SQL = " select * From (select tag_id,TAG_VAL,   row_number()over(partition by TAG_ID order by CLOCK DESC) rn from T_PP_OPC_INPUT s1 where (TAG_ID >= 96 )   ) a where rn='1'";

My table structure following.
RUN_ID    STRING
TAG_VAL   STRING
TAG_ID    STRING
CLOCK     CLOCK

your early reply would be appreciable..

thanks in advance.