The date of expiration of the account, expressed as the number of
           days since Jan 1, 1970.

           Note that an account expiration differs from a password expiration.
           In case of an acount expiration, the user shall not be allowed to
           login. In case of a password expiration, the user is not allowed to
           login using her password.

           An empty field means that the account will never expire.

           The value 0 should not be used as it is interpreted as either an
           account with no expiration, or as an expiration on Jan 1, 1970.