Just to avoid the confusion, you need to book on the night of 9th of December ( Sunday) at 0000 hours when date changes to 10th
if you want to book for 21st of Jan 2013 !
Due to high demand and over booking, dates get booked within few minutes after the midnight so it is advisable to try at midnight. Though, cancelled/reshdeduled slots can appear any time during the day but it is merely a lucky dip.
BTW, 28 days does not apply to dependents they must complete 2 years before the appointment day, so in your case, this date is wrong, IMHO. I am sure I have seen such posts on the forum, try forum search.