The U.S. government will hit the $16.4 trillion federal debt limit Monday and turn to “extraordinary measures” to continue borrowing, the Treasury Department said Wednesday, beginning a countdown until Congress either passes legislation to allow for more borrowing or the government defaults on its debt.

In a letter to Congress, Treasury Secretary Timothy F. Geithner said that although the debt ceiling would be reached Dec. 31, the government could buy roughly two months’ more time before it would be unable to meet all its obligations.