Discovery bank false advertising.

Discovery advertises their credit card as 55 days interest free. On the 5th of December I used R35000 on my credit card wich I paid back in full on the 23rd December. I was charged R42.31 interest on the 7th of December and R338.46 interest on the 8th of January. Upon calling to enquire I was informed that because the statement time is from the 7th of the month till the next month on the 8th my account was in a minus and thus I was charged interest. So I was charged interest from 2 days after I used my credit card. Absolutely rediculous! They also cannot call you back from the call centre or provide you with an email adress to file a complaint.