The nearest expiration date for Dated Brent (Platts) to Frontline Brent Futures (Dec 2013) is Dec 31, 2013.
The best time to sell a futures contract is when you've already made money on the investment, but still have plenty of time left before the expiration date. Thus, a good option would be to sell Dated Brent (Platts) to Frontline Brent Futures (Dec 2013) before Dec 31, 2013.