Changing Date Time Format in Word 2007

Having chosen a particular date/time format and used it in several macros, I now can't get Word to let me change that date/time format for other functions.

Now, when I go to INSERT/Date & Time the drop down menu where one can select a particular format doesn't appear; instead Word just inserts the date and time in the same format I've used before.

Any suggestions?

Thanks,

Terance