Fifth Third Bank 5/3 Bank Charging interest on 0% offer Cincinnati Ohio

I received an offer from 5/3 for a balance transfer to my 5/3 credit card. The offer included a 0% interest rate for 18 months. It expired Sept 30. On Sept 28, I paid my 5/3 credit card balance in total so I would be starting with a 0 balance when I did the new transfer. On Sept 30, I spoke with a representative by telephone to complete the balance transfer using the promotion. On Oct 21, an interest charge posted to my account on the balance. Today I called to speak with a representative. She said she spoke with her supervisor who said that the promotion expired on Sept 30 so I would have had to transfer on Sept 29. I said that is not my understanding of how expiration dates work. And why would it expire on not the last day of the month? So I asked to speak to someone else. I then spoke with a supervisor who submitted a dispute for me. So now I have to wait for them to review it and get back to me.