Default is not in our stars, Horatio, but in ourselves.

Yellen has updated the X-date when the US will technically default. The Treasury expects to exhaust its extraordinary measures potentially as early as June 1.

Tax receipts have been below trend this year, so this isn't entirely unexpected.

She's likely being somewhat conservative here. Some of the extraordinary measures are harder to predict than others. But as she notes even if we avoid default, this brinkmanship isn't without costs. The 2011 debt limit showdown caused a credit rating agency to downgrade the US government, and increased the Treasury's borrowing costs by an estimated ~$19 billion.