Hi, webmax!
Data in column A has format dd/mm/yyyy hh:mm, so for A2 (12/05/2013 21:58:11) you'd expect May 12th, regardless the format of destination cell (B2, in the example) which is dd-mmm-yy, but you say you want Dec 5th.
So you're actually trying to invert cell contents with DMY to MDY but formatted as DMY, i.e, swap day and month? Could you please confirm this, since depending on the day values of the data it'll lead you to value errors.
Could you post too other examples with day greater than 12?
Regards!