Incorrect information

So, as a consumer, i feel that proper advice should be given. When i took out my Markham account, i was getting paid Month end so it made sense for me to have a debit order date on the 2nd to be safe. I was told that if i pay my account within 6 Months its Interest free? Yesterday i queried the issues and i was informed that because my debit order is the 2nd, its seen as a late payment? Surely this is unfair? If i was told this from the beginning i would have made sure that my debit order be noted as the 1st? Its almost 4 years of Interest that i was charged and had absolutely no clue about as its not even noted on the statement.