To Find the difference between two dates

The answer is hidden in the documentation for LAST_DAY:
http://docs.oracle.com/cd/B19306_01/server.102/b14200/functions072.htm#i83733[^]

From there:

SELECT SYSDATE,
   LAST_DAY(SYSDATE) "Last",
   LAST_DAY(SYSDATE) - SYSDATE "Days Left"
   FROM DUAL;

The result is:

SYSDATE   Last       Days Left
--------- --------- ----------
30-MAY-01 31-MAY-01          1

See the third line? You just subtract one date from another.
So you do SELECT DATE1 - DATE2 FROM DUAL and get a number, which is a number of days.
Divide by seven, you'll get the weeks.

Here is a little test:

SELECT 
  SYSDATE - TO_DATE('2012-03-01', 'YYYY-MM-DD') "Time In Month",
 (SYSDATE - TO_DATE('2012-03-01', 'YYYY-MM-DD')) / 7.0 "Weeks" 
from dual;

And the result:

Time In Month          Weeks                  
---------------------- ---------------------- 
25.659                 3.666                  

1 rows selected

To check the months, I'd suggest this:

SELECT TO_DATE('2012-01-01', 'YYYY-MM-DD') FROM DUAL;
SELECT SYSDATE, TO_DATE('2012-01-01', 'YYYY-MM-DD') "TEST_DATE", 
       ADD_MONTHS(TO_DATE('2012-01-01', 'YYYY-MM-DD'), 4) "Test + 4 months", 
       CASE WHEN ADD_MONTHS(TO_DATE('2012-01-01', 'YYYY-MM-DD'), 4) > SYSDATE THEN 'Less than 4 months ago'
       ELSE 'More than 4 months ago'
       END /* CASE */ "Test result"
   FROM DUAL;

The results are:

TO_DATE('2012-01-01','YYYY-MM-DD') 
------------------------- 
01-JAN-12                 

1 rows selected


SYSDATE                   TEST_DATE                 Test + 4 months           Test result            
------------------------- ------------------------- ------------------------- ---------------------- 
26-MAR-12                 01-JAN-12                 01-MAY-12                 Less than 4 months ago 

1 rows selected

Hope this helps,
Pablo.