This Rail Panel report gives a date of 10 December for both the opening of Langdon Park station and the long delayed use of both new platforms at Stratford, a date also mentioned in this news article (thanks to Abigail and Bob for sending in those links).
Complicating things, diamondgeezer sends in the above photo, which says the timetable will be changing on 8 December, which would presumably be to accommodate both developments.
Hopefully we'll get an official announcement soon of the exact date.