Try this one I hope will help you, commented the part that compares the dates and shows error message in your code.

trigger oppMasterLeaseAgreementUpdate on Opportunity (after insert, after update, after delete, after undelete) {
    Set<String> RECORDTYPE_NAMES = new Set<String>{'AUS : CFS : Opportunity Record Type', 'AUS : DFS : Opportunity Record Type'};
    
    OppUtilities.isOppTrigger = true;
    
    Opportunity[] records;

    //Get the records
    if (Trigger.isInsert || Trigger.isUndelete || Trigger.isUpdate) 
        records = Trigger.new;
    else if (Trigger.isDelete) 
        records = Trigger.old;

    
    Map<String, Id> recordTypeMap = oppUtilities.getOppRecordTypeMap();
    
    Set<Id> recordTypeIds = new Set<Id>();
    
    for (String recType : RECORDTYPE_NAMES) {
        System.debug(recType + '::' + recordTypeMap.get(recType));
        recordTypeIds.add(recordTypeMap.get(recType));
    }

    List<Id> mlaIds = new List<Id>();
    
    Map<Id, Map<Id, Datetime>> mapValidateDate = new Map<Id, Map<Id, Datetime>>();
    
    //Loop through the opportunity records
    for (Opportunity record : records) {        
        if (recordTypeIds.contains(record.RecordTypeId)) {
            if (!(Trigger.isUpdate && Trigger.oldMap.get(record.Id).Amount == record.Amount 
                    && Trigger.oldMap.get(record.Id).Master_Lease_Agreement__c == record.Master_Lease_Agreement__c
                    && Trigger.oldMap.get(record.Id).ForecastCategory == record.ForecastCategory)) {
                
                if (record.Master_Lease_Agreement__c != null) {
                    System.debug('debug1:' + record.Master_Lease_Agreement__c);
                    mlaIds.add(record.Master_Lease_Agreement__c);
                    
                    //we need to store the created date because we are validating whether the it is greater than the MLA Credit_Expiry_Date__c
                    mapValidateDate.put(record.Id, new Map<Id, Datetime>{record.Master_Lease_Agreement__c => record.CreatedDate});
                }
                
                //In case the user changes the Master_Lease_Agreement__c field from one Master_Lease_Agreement__c Id to another, update the old one too
                if (Trigger.isUpdate && Trigger.oldMap.get(record.Id).Master_Lease_Agreement__c != record.Master_Lease_Agreement__c 
                        && Trigger.oldMap.get(record.Id).Master_Lease_Agreement__c != null) {
                    System.debug('debug2:' + Trigger.oldMap.get(record.Id).Master_Lease_Agreement__c);
                    mlaIds.add(Trigger.oldMap.get(record.Id).Master_Lease_Agreement__c);
                }
            }
        }
    }
    
    if (mlaIds.size() > 0) {
        OppUtilities oppUtil = new OppUtilities();
        
        Map<Id, Decimal> totalWonAmountMap = new Map<Id, Decimal>();
        Map<Id, Decimal> totalOpenAmountMap = new Map<Id, Decimal>();
        
        totalWonAmountMap = oppUtil.getTotalOpportunityAmountsByMLAIds(mlaIds, OppUtilities.ForecastType.WON);
        totalOpenAmountMap = oppUtil.getTotalOpportunityAmountsByMLAIds(mlaIds, OppUtilities.ForecastType.OPEN);
            
        Map<Id, Master_Lease_Agreement__c> mapMLAs = new Map<Id, Master_Lease_Agreement__c>([SELECT Id, Name, Credit_Expiry_Date__c FROM Master_Lease_Agreement__c WHERE Id IN :mlaIds]);
            
        //loop through opps and throw add error to records that are linked to Master_Lease_Agreement__c that has Credit_Expiry_Date__c in the past
        for (Id oppId : mapValidateDate.keySet()) {
            Map<Id, Datetime> mapValidateDateInner = mapValidateDate.get(oppId);
            
            Id mlaId;
            
            //This inner map will always have 1 key-value pair in it, the mla Id related to the opp and the opp Created Date
            for (Id id : mapValidateDateInner.keySet()) 
                mlaId = id;
            
            Datetime oppCreatedDate = mapValidateDateInner.get(mlaId);
            
            System.debug('MLA Credit_Expiry_Date__c:' + mapMLAs.get(mlaId).Credit_Expiry_Date__c);
            System.debug('Opp Created Date:' + oppCreatedDate);
            /*if (Date.newInstance(oppCreatedDate.year(), oppCreatedDate.month(), oppCreatedDate.day()) > mapMLAs.get(mlaId).Credit_Expiry_Date__c)
                Trigger.newMap.get(oppId).addError('This Opportunity with Id=' + oppId + ' cannot be linked with Master Lease Agreement ' + mapMLAs.get(mlaId).Name
                                                        + ' because the Created Date of the Opportunity is past the Credit Expiry Date of the Master Lease Agreement');*/
        } 
            
        //Update the Master_Lease_Agreement__c records' Line_of_Credit_Used__c and Line_of_Credit_in_Pipeline__c fields
        for (Master_Lease_Agreement__c mla : mapMLAs.values()) {
            mla.Line_of_Credit_Used__c = totalWonAmountMap.get(mla.Id);
            mla.Line_of_Credit_in_Pipeline__c = totalOpenAmountMap.get(mla.Id);
        }
        
        update mapMLAs.values();
    }
    
}