The tweet has since been deleted, although IGN has a screenshot. We have a partial screenshot taken from GAME Brighton's Facebook page which backs this up, with the tweet likely being automated to the outlet's profile (again the FB message is now deleted):
Across the weekend, a Polish fansite
put up alleged posters for the game, also indicating a Spring 2013 release. To reiterate, Rockstar has not made any official announcement about the game's release date.