Avis manipulate time to charge me extra.

Good Day<br> <br> On the 29th of April I rented a car from Avis - Veeriniging. Rental no. ***. I agreed to return a car on the 30th at 15h00. I was told that I have 29 min grace period before being charged extra. I returned a car at 15:28, thus within my grace period. But I was still charged for an extra day. When I enquired why, the lady told me I was suppose to return a car at 14:50 and have went above my 29 minutes grace. <br> <br> Why did Avis write 15H00 on my agreement if they wanted me to return the car at 14H50? I think I was treated unfairly and deserve to be refunded. Why does Avis manipulate times to charge me extra?<br> <br> I have copies of both the agreement and the return document .<br> <br>