Having trouble converting from english date!

When i try and re-arrange a date from english dd/mm/yy to yy/mm/dd excel won't automatically do this if the day is more than 12. e.g. If i tried to change 10/12/01 it would change it to 2001/12/10 fine, when i try the same with 13/12/01 it won't work. Any advice? I know it can be done manually but i have to change over a thousand dates this way so i'm using a macro.