The DateTime.ToString method is case sensitive: "MM" is not the same as "mm".
The first one is the Month, with a leading zero, the second is the minute with a leading zero.
If you want the date, you need to look at what ThePhantomUpvoter said, and change your "d/mm/yyyy" to "dd/MM/yyyy"
There is a list of format codes here which might help:
Formatting a DateTime for display - format string description[
^]