Although nominations for the 2013 Hugo Award will remain open though March 10, you must become a member of LoneStarCon 3 (the 2013 World Science Fiction Convention, Loncon 3 (the 2014 Worldcon), or already be a member of Chicon 7 by January 31, 2013 in order to be eligible to nominate for the 2013 Hugo Awards.
Anyone who is a member of one of those three Worldcons by the end of January may cast a nomination ballot for the 2013 Hugo Awards. Ballots must be received by Sunday, March 10, 2013, 11:59 p.m. EDT (Sunday, March 10, 2013, 8:59 p.m. PDT; Monday, March 11, 2013, 3:59 a.m. UTC/GMT).
After the finalists are announced later this year, all Supporting, Attending, and Young Adult members of LoneStarCon 3 (including all members who join prior to the closing date of the final ballot) will be invited to submit ballots to select the Hugo winners. LoneStarCon 3 members will also be eligible to nominate for the 2014 Hugo Awards, to be hosted next year by Loncon 3, the 72nd Worldcon, in London, UK.