Second attempt, now with no switch statements!

DateLogicOppRound2.cls

trigger DateLogicOpp2 on OpportunityLineItem (before insert, before update) {
	  for (OpportunityLineItem oli :Trigger.new) {
        
       if(oli.ServiceDate !=null){
        
           DateTime startDate = DateTime.newInstance
           					   (oli.ServiceDate, Time.newInstance(12,0,0,0));

           if (oli.Materials_Due_2_Weeks__c) {
           oli.Materials_Due_Date__c = oli.ServiceDate - (Integer.valueOf(startDate.format('u')) + 11);
           } else {   
           oli.Materials_Due_Date__c = oli.ServiceDate - (Integer.valueOf(startDate.format('u')) + 4);
           }
           
           oli.Internal_Due_Date__c = oli.Materials_Due_Date__c.addDays(-2);
           
           
}
      }
    
}