[support] is it possible to have validation using CCK fields?

hey there - I'm looking to use CCK fields for defining if an article is exclusive to a role of users along with a specified timeframe. Is it possible to utilize some sort of validation whereby if a role is specified then a startdate and enddate must be specified? The trick is that if they specify a role, I need them to also specify a startdate and enddate. Does the CCK module allow for hooks to alter the form and/or specify more advanced validation rules? i've read about node_alter_form and hook_alter_form, but how does one use them with CCK fields? Any advice would be greatly appreciated. regards, brian