Passwords are important for your security. It is important that they remain a secret. Over time they can leak out. To enhance your security passwords 'expire' after one year.
For security reasons, passwords for the Windows network expire one year after they were last changed. After a password has expired it can no longer be used and you will be locked out of your account.
ISS provide two password expiry reminder systems:
For a 'reasonable period' it is possible to change your password AFTER your password has expired.
Change Password - For advice about changing password (including instructions).