by Jim Rickards, Daily Reckoning:

The “X-Date” is the day the U.S. Treasury goes broke. It’s not the same as the debt ceiling but it is a consequence of the failure of Congress to raise the debt ceiling.

Right now, the Treasury is at the debt ceiling. That means it has no legal authority to issue net new debt. It can issue new debt if old debt is maturing. That keeps the total debt unchanged; you’re just rolling over maturing debt into new debt without increasing the total debt.