DAMAGED CELL PHONE

I bought a cell phone on the 18th May 2015 at 17:46. Now the cell phone has the problem with the dialling pad and the speaker. I went back to the store on the 25th May 2015 for an exchange, as it is stated on the slip that the handset can be exchanged if returned within 7 days. T he lady told me that the 7 days has elapsed and they cannot exchange the phone. She said the 7th day was the 24th May 2015.