In the course of any given month, the government collects billions of dollars in taxes, spends billions more, and borrows money to cover the difference between what it collects and what it spends.
If Congress doesn't raise the debt ceiling very soon, the government won't be able to borrow money to cover the difference anymore, and won't be able to pay all of its bills.
Treasury Secretary Jack Lew has said that, after Oct. 17, the government can't guarantee that it will be able to make all its payments. But the government won't run out of money on the 17th; it will still have about $30 billion left.
The Bipartisan Policy Center predicts the money will run out some time between Oct. 22 and Nov. 1. This graph shows some major payments the government has to make during that window. Note the pile of bills due on Nov. 1.
It's unclear exactly when the government will run out of money. That's because the amount of money the government collects in taxes on any given day can vary a lot. Spending can vary, too.
As the Bipartisan Policy Center wrote recently:
For example, in October of last year, about $3 billion in revenue was collected on one Tuesday, just over $6 billion was received on another Tuesday, and almost $11 billion arrived on yet another Tuesday. Furthermore, while spending is subject to less uncertainty than revenue, it still varies. For example, on some days, Medicare spending can exceed $2.5 billion while on other days, it is closer to $1 billion.
NPR's John Ydstie has more on this story today on Morning Edition.