How to find out when did a user changed his password in oracle 11g

Summary :-
The table sys.user$ contains the field ptime, which keeps the time when the password was
changed the last time over. Do not confound it with ctime, which is the "creation time", nor with
ltime, which is the time the account has been locked (if any).

SQL>SELECT NAME, ptime AS "LAST TIME CHANGED", ctime "CREATION TIME", ltime "LOCKED"
FROM USER$
WHERE ptime IS NOT NULL
ORDER BY ptime DESC;


NAME                           LAST TIME CREATION  LOCKED
------------------------------ --------- --------- ---------
APPUSER                        27-FEB-15 16-FEB-15
HELPDESK                       27-FEB-15 27-FEB-15
SOUMYA                         27-FEB-15 14-FEB-15
SYS                            25-FEB-15 15-AUG-09 15-AUG-09
SYSTEM                         25-FEB-15 15-AUG-09 15-AUG-09
BRIAN                          16-FEB-15 16-FEB-15