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trigger updatePeriodeByCloseDateTrigger on Opportunity (before insert, before update) {
    for (Opportunity opp : Trigger.new) {
        if(opp.CloseDate != null) {
            if(Trigger.isInsert) {
                opp.Periode__c = DateUtil.closeDateToStringMonthYear(opp.CloseDate);
            }
            else if(Trigger.isUpdate) {
                opp.Periode__c = DateUtil.closeDateToStringMonthYear(opp.CloseDate);
            }
        }
    }
}