And all under Joe Biden.

"Once all available measures and cash on hand are fully exhausted, the United States of America would be unable to meet its obligations for the first time in our history," Yellen wrote.

"Based on our best and most recent information, the most likely outcome is that cash and extraordinary measures will be exhausted during the month of October," Yellen continued.

She went on to lay out why this is a big deal, noting that delaying now could "cause serious harm to business and consumer confidence, raise short-term borrowing costs for taxpayers, and negatively impact the credit rating of the United States.

"A delay that calls into question the federal government's ability to meet all its obligations would likely cause irreparable damage to the U.S. economy and global financial markets," she wrote.