Hi Tommy,

When you want to take into account your business hours (9AM - 5PM), it is always a tricky problem because the "real" business hours take also into accounts the hollydays and the timezones.

The most accurate and "easy" solution is to use a trigger here.

One line of code is sufficient in Apex and all the details of business hours are take into account directly.

Double First_Response_Time = BusinessHours.diff(defaultBusinessHours.Id, opp.CreatedDate, OppMinEvtCreateddate)/3600000.0;

Use the BusinessHours methods to set the business hours at which your customer support team operates.


 

trigger EvtCalculateBusinessHours on Event (after insert, after update, after delete) {   
    Set<Id> oppIds = new Set<Id>();
    
    //Get the default business hours 
    BusinessHours defaultBusinessHours = [select Id from BusinessHours where IsDefault=true];
    
    Map<Id,DateTime> mOppMinEvtCreateddate = new Map<Id,DateTime>(); 
    Map<Id,DateTime> mOppMinStartdatetime = new Map<Id,DateTime>(); 
    
    List<Event> evts = null;
    
    if (Trigger.isInsert || Trigger.isUpdate) {    
        evts = Trigger.new;
    } else {
        if (Trigger.isDelete) {     
            evts = Trigger.old;
        }
    }
    
    if(evts != null) {
        for (Event evt:evts) {
            if (evt.WhatId != null) {                     
                String sobjectType = evt.WhatId.getSObjectType().getDescribe().getName();
                if (sobjectType == 'Opportunity') {
                    oppIds.add(evt.WhatId);
                    // default null date in case of deleting the last event
                    mOppMinEvtCreateddate.put(evt.WhatId,null);
                    mOppMinStartdatetime.put(evt.WhatId,null);
                }
            }
        }
    }
    
    if (oppIds.size() >0) {
        // Get the min event dates for all the opportunities
        AggregateResult[] groupedResults = [select WhatId, min(createddate) minCreateDate, min(startdatetime) minStartdatetime from Event where whatId in :oppIds group by whatId] ;
        for (AggregateResult ar : groupedResults)  {
            System.debug('What ID:' + ar.get('WhatId'));
            System.debug('Min CreatedDate:' + ar.get('minCreateDate'));
            mOppMinEvtCreateddate.put((Id)ar.get('WhatId'), (DateTime)ar.get('minCreateDate'));
            mOppMinStartdatetime.put((Id)ar.get('WhatId'), (DateTime)ar.get('minStartdatetime'));
        }  
        // Get all the created date for all the opportunities sent to the trigger
        List<Opportunity> opps = [select id,createddate from opportunity where id in :oppIds];
        
        List<Opportunity> oppsUpdated = new  List<Opportunity>();           
        for (Opportunity opp:opps) {              
            // The diff method comes back in milliseconds, so we divide by 3600000 to get hours.
            // diff(businessHoursId, startDate, endDate): returns the difference in milliseconds between a start 
            //   and end Datetime based on a specific set of business hours                
            DateTime OppMinEvtCreateddate = (DateTime)mOppMinEvtCreateddate.get(opp.Id); 
            DateTime OppMinEvtStartdatetime = (DateTime)mOppMinStartdatetime.get(opp.Id); 
            if (OppMinEvtCreateddate == null) {
                oppsUpdated.add(new Opportunity(id=opp.Id,First_Response_Time__c=null));  
            } else {
                system.debug('opp.CreatedDate :' + opp.CreatedDate);
                system.debug('OppMinEvtCreateddate :' + OppMinEvtCreateddate); 
                system.debug('OppMinEvtStartdatetime :' + OppMinEvtStartdatetime); 
                Double First_Response_Time = BusinessHours.diff(defaultBusinessHours.Id, opp.CreatedDate, OppMinEvtStartdatetime)/3600000.0;
                System.debug('First_Response_Time:' + First_Response_Time);
                oppsUpdated.add(new Opportunity(id=opp.Id,First_Response_Time__c=First_Response_Time));  
            }                     
        }
        if (oppsUpdated.size() > 0) {
            update oppsUpdated;
        } 
    }   
}

The remaining problems:

1. You need two triggers: one for the events and one for the tasks (the same as above replacing just Event with Task)
2. Only one definition of business hours has been used above (the default one) and that could be not sufficient.
3. Each record of Case has its own BusinessHoursId but not the Event object. BusinessHours is closely linked with the cases.
4. You must initate all the pre-existing opportunities for the First_Response_Time__c field (empty by default)

The initialization of the First_Response_Time__c field for all the opportunities is not difficult if you have the "dataloader" tool.

How many events are there in your org?