Credit Card doubled up due to FNB error

My billing cycle for my FNB credit card runs from the 6th to the 6th of the month and has done so for probably 20 years plus. This month FNB ran my cycle form the 6th to the 26th with no warning or advance notice resulting in a double up of the month end debit. I called them and the call centre were unwilling to help me. I am disgusted and flabbergasted by my treatment and feel that this is borderline ***** committed by the bank. I have never been treated so dishonestly by FNB and I am reporting the issue here as no one at the bank wants to take accountability and fix this.