Set Expiry Date based on value of another property

Hi,

Hope someon can point me in the right direction.

One on screen I have a dropdown list for EmployeeType attribue (i.e. Perm, Temp).

On another tab is the Expiry Date.

I want to make sure that if Perm is selected for Employee Type that expiry date is left as Never (greyed out, can't be changed would be idea).

If Temp is set then Never is not allowed to be used, rather a future date has to be set.

Please can someone advise how I would do this ?

Thanks

Andy