There are a few things that are possible but first a question
Why have that format on the database level? Best way to have your app to work allways is to set it on yyyy-mm-dd, and change it in code to what you want.
Your regional settings have changed : do a SHIFT +ALT
The easy thing to do is type in as format yyyy mm dd so 2007-12-01
and this will change by lostfocus to database format 01-DEC-2007