How to turn off auto-reformatting of dates?

I recently upgraded to iWork '09.

I have a files saved from prior versions of Numbers. I happen to format my dates as:

YYYY-MM-DD

It is the manner I prefer as it keeps them chronological when I sort (not specific to Numbers, but just in general -- for instance, folder names and such). I always write my dates this way and prefer that they stay this way.

Now, when I open my Numbers files in iWork '09, it keep pre-existing cells in the date-format as I have them, but when I try to add a new cell, and I type the date in as, for instance, "2010-02-07" and I tab or select to the next cell, it auto-reformats the cell to read, "Feb 7, 2010".

I cannot find any way to turn this off.

The file I happen to be working on goes back several years. I don't want new date fields to suddenly switch from my preferred formating to this new formating beginning this week.

Thank you for any assistance in turning off this new "feature."

Thanks.