Treasury Secretary Mnuchin sent Congressional leadership a letter today informing them the Treasury has launched extraordinary measures until Congress raises or suspends the debt limit, making true on his promise from last week. The U.S. reached its debt cap yesterday, and the national debt is currently $19.918 trillion, according to Forbes. So in the meantime, the Treasury is allowed to reorganize its government accounts to reduce intragovernmental debt.
But that can't go on forever: The CBO estimates the measures will be exhausted "sometime in the fall of this year without an increase in the debt limit, though an earlier or later date is possible." Cash inflows from tax season will also sustain the Treasury for a few months, according to the CBO.