This post has been updated.

As promised, Treasury Secretary Timothy Geithner on Monday night formally notified Congress that the United States has reached its $16.4 trillion debt ceiling.

In a letter to lawmakers, Geithner wrote that the government will take extraordinary measures to extend the deadline by which Congress must act to raise the borrowing limit. The Treasury Department noted earlier this month that while the roughly $200 billion in additional headroom would ordinarily last about two months, the uncertainty surrounding taxes and spending policies as a result of the fiscal cliff talks means it's currently not possible to calculate the actual deadline.

Read Geithner's letter here.