First Enter 12 passwords then change the system date and 
give the date such that the difference is about 30 days 
then try to enter the password that you have given 
previously ( between all the 12 ) if it accepts then its a 
bug.

To check the Password on diffarent dates you need to change
the appropridate system date ,here are some follwing Testcasess

Objective is login Expire
--------------------------

DESC                                          Expected O/P
--------                                      -----------
1)Check the password on any day with in 30--> should login
2)Check the password on 30 th day--->should login
3)Check the password on 31 th day--->Should not login
4)Update the Password after 20 th day then 
check after 20 days--> should login
5)Update the Password after 50 days(Expire) then check the 
password after 20 days--> Should login
6)Update the Pasword of another user after 20 days then try
to login after 31 days--> Should not login

Objective is should not allow last 12
---------------------------------------

DESC                                          Expected O/P
--------                                      -----------
1)Check by updating as previous Password--> should not allow
2)Check by updating same Password-->should not allow
3)Check by updating as 12 th password--> should not allow
4)Check by updating as 13 th password-->should allow
5)Check by updating as new Password-->should allow
6)Check by Updating some other user previous
passowd-->should allow

Etc