Spending thresholds are based on the calendar year, not posting date. If you hit the spend threshold in the calendar year, the MQM's will post and count towards 2013 totals regardless of when the statement closes or when your payment is due. They will retroactively add the miles back towards 2013, bump your status, and adjust your rollover down to 5K.
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FYI they actually backdate the transaction so it will be as if you hit DM on the 31st, BUT the system just will not reflect that, until after your statement posts.
http://www.flickr.com/photos/4520422...7283/lightbox/
My statement closes on the 3rd. So you will see that the miles for that month posted on the 4th, but the bonus is backdated to the 12-31. Then, on the 6th, the rollover occurred, backdated to 1-1.