if you mean how will I insert 31/12/2003 type date into a date field, you won't , you need to format it as a mysql style date to be able to use all of myslqs date functions on the field.
In this case I normally add a varchar(10) field (old_date) and a DATE field (new_date), and then convert the date from the old format to mysql format.
update my_table set new_date=date_format(concat(@year:=substring(old_date ,7,2),@month:=substring(old_date ,4,2),@day:=substring(old_date ,1,2)), '%Y-%m-%d');
*note, This is for dd/mm/yy but you can ove the elements around in the UPDATE query.
** Added this as a FAQ at long last
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There's no present like the time, they say. - Henry's Cat.