In a scenario i want to retain only the old values of the event if the event is updated directly when related to a custom object. Here is my  trigger. IT works fine for all the event fields except event start date and end date. These two field are not retaining their old values, It accepts the user Inputs.Also Debug statements working fine as expected. But its not updating the field values.

trigger prohibitUpdate on Event (before update) {
Set<id>neweveid=new set<id>();
Set<id>whatid=new set<id>();
for(event eve:trigger.new){
neweveid.add(eve.id);
whatid.add(eve.whatid);
}

for(event eve:trigger.new){

Event oldEve = Trigger.oldMap.get(eve.id);
if(trigger.isupdate){
system.debug('******'+trigger.old[0].StartDateTime);
          eve.StartDateTime=oldEve.StartDateTime;
          eve.EndDateTime=oldEve.EndDateTime;
          eve.Onsite_Arrival_DateTime__c = oldEve.Onsite_Arrival_DateTime__c;
          eve.Onsite_Completion_DateTime__c =  oldEve.Onsite_Completion_DateTime__c;
          eve.Location = oldEve.Location;
          eve.Description = oldEve.Description;
          eve.OwnerId = oldEve.OwnerId;
          eve.ShowAs = 'OutofOffice';
          eve.Subject = oldEve.Subject;
          eve.WhoId = oldEve.WhoId;       
}
}
}