And the bills will exceed the revenue and cash at some point after that. So something will have to give. Someone, perhaps seniors due their Social Security payments, won't get paid on time. It's that simple.
Stocks will probably plunge. The U.S. government's incredibly cheap borrowing costs could jump. In a worst-case scenario, the gears of the financial system could gum up because Treasury debt is such an important lubricant.
Some economists fear a jobs-killing recession if a debt ceiling crisis persists. That's because the effective result of not raising the debt ceiling would be a massive and abrupt cut in federal spending.
The only good news is that this latest Washington-made crisis has an obvious on/off switch. It's not too late -- lawmakers can flip the "off" switch now.
But will they? Longtime budget expert Stan Collender, once a Democratic staffer on Capitol Hill, says it's a coin flip that Congress will raise the debt ceiling by Thursday -- a 50/50 chance.
"One of the biggest problems with the current shutdown/debt ceiling situation is that no one has any assurance that the person they're negotiating with has any authority to agree to anything," Collender wrote Sunday.
And the Treasury market has shown warning signs in recent days, as buyers of some short-term bills have demanded higher rates.
CEOs have started warning about stresses their businesses would face in a worst-case scenario.
China: Solve debt ceiling ASAP
It's hard not to flash back to five years ago.
On this day in 2008, it was one month since Lehman's bankruptcy, and global credit was frozen.
Stocks had been pummeled but, on October 13, soared a dizzying 10%. Treasury was getting closer to deploying the new $700 billion TARP bailout Congress had narrowly approved. Plus, the Federal Reserve and other central banks were working 24/7 pumping money into the system.
So investors thought, maybe the worst was over.
Of course, it turned out there were many more dark moments in the months ahead. Citigroup nearly failed. The government jumped in to lead the bankruptcies of General Motors and Chrysler. Worst of all, there came report after report of job cuts that devastated the economy and upended lives.
Five years later, the banks and economy are far stronger. Congress may test just how much stronger if it fails to raise the debt ceiling.