password expiration field set to none after password change

[peter sands]

Is there a way to set the 'password expiration'  field  with modprinc
( pwexpire) to be constant.
Currently I've set it at @ 30 days.
When this date is reached , the user changes their expiring  password,
which is all good.

However  the  password expiration field is then reset to 'None':
Password expiration date: [none]

I have a script that goes round and changes the expiration for another
30 days, so that's OK. But is there a way the value for password
expiration can be constant and not reset.

(using aix nas/kerberos 5)

Thanks
pete