Format issue with dates over the 13 in excel

Format issue with dates over the 13 in excel

I have the following problem.
I'm not able to modify all dates older than the day 13.

For example:
With the date 07/01/2013 11:20:00 I am able to change the format. To
07/01/2013 11:20 a.m.
But with 07/13/2013 00:00:00 or over the 13, I'm not able to change it to
anything.

Even if I manually change the format, the date won't change.
Do you know what could be wrong?
Thanks