FWIW, I encountered this problem myself just this morning on a relatively new installation. When I started to type "Wednesday," the AutoComplete suggestion was "Wednesday, December 05, 2012," which was, of course, not what I
wanted. When I looked at the Date and Time dialog (this is in Word 2003), "Wednesday, December 5, 2012" was not one of the options. So I went to the Region and Language folder in Control Panel (Windows 7), and that was not one of the options in the short list
there, either (as I suspected, "dddd, MMMM dd, yyyy" was currently selected as the default Long Date format). It is necessary to click the button for Additional Settings... and then select the Date tab, where the date format can then be customized. As soon
as I made the change and closed Control Panel and returned to Word, the Date and Time dialog reflected the change.
Thanks. As mentioned in my original post, I always have used Region and Language in Control Panel to set the correct format for dates. Only very recently has it not worked. Here is the date in Region and Language after I corrected it:
dddd, MMMM d, yyyy
And yet after closing Control Panel and opening a new page in Word it still reads:
Wednesday, December 05, 2012