The Metropolitan Transportation Authority has long said December 2016 is when the Second Avenue Subway will open, but the Federal Transit Administration isn't so sure.
The agency says phase one of the new subway line will actually open at the end of February 2018.
The head of the FTA reiterated that date Monday at a congressional hearing in Manhattan.
The federal agency also says it will cost a billion dollars more than the MTA estimates, $5.5 billion instead of $4.5.
Recently the head of MTA Capital Construction said the project is more than 80 percent done, and described it as "on time and on budget."
An MTA spokesman says: “The MTA reports our projections for megaproject cost and completion every month based on our own understanding of the work done so far and our best estimates of the work still to be done.”
The MTA has been criticized for cost overruns and delays on projects like the 7 train extension and East Side Access, which will bring the Long Island Rail Road to Grand Central.