The $28.4 trillion debt limit was reinstated August 1. Since then, Treasury Secretary Janet Yellen has been keeping the nation's finances afloat by using emergency accounting maneuvers. Known as "extraordinary measures," these steps allow the government to borrow additional funds without breaching the debt ceiling.

But Yellen warned lawmakers this week that if Congress fails to raise, or suspend, the debt ceiling, the federal government will exhaust those extraordinary measures by October 18.

"At that point, we expect Treasury would be left with very limited resources that would be depleted quickly," Yellen wrote in a letter to Congress. "It is uncertain whether we would continue to meet all the nation's commitments after that date."

That's an accelerated timing from previously, when Yellen said this would happen at some point in October.

This so-called X-date should be viewed as a best guess by Treasury, not a set-in-stone deadline based on exact science. In other words, America could hit the debt ceiling days before, or days after, October 18.

There are many, many moving pieces here. Yellen herself warned the X-date "can unpredictably shift forward or backward."

And given the stakes — Jamie Dimon has warned a default would cause a "cascading catastrophe of unbelievable proportions and damage America for 100 years" — it would be risky for Congress to get too close to the X-date, let alone beyond it.

Is there any leeway? Maybe

Of course, Treasury secretaries have a long history of giving themselves wiggle room when announcing an X-date — just in case Congress blows through that date. That's what happened in 2011: Washington didn't default then, but the delay did cost America its perfect credit rating from S&P and rattle financial markets.

It's the same idea as telling someone who is chronically late that dinner is two hours earlier than scheduled -- just to make sure they're on time.

There is some debate over just how much wiggle room Yellen is giving Congress this time.

Yellen was asked at a hearing on Thursday "what happens on October 19" and her response indicates there could be a bit of leeway, but not much.

"We're simply in an impossible situation in which it will be impossible for Treasury on that day, or a few days thereafter...We'll have very limited resources," Yellen said. "It will be run down quickly. We won't be able to pay all of the government's bills."

However, the nonpartisan Congressional Budget Office issued a report Tuesday that suggests there may be additional time beyond October 18 for lawmakers to avoid a catastrophic default.

The CBO projects that if the debt limit remains unchanged, Treasury's ability to borrow using extraordinary measures will be exhausted and it will "most likely run out of cash near the end of October or the beginning of November." That's unchanged from CBO's prior estimate in July.

CBO warns that after that happens, the federal government would be "unable to pay its obligations fully, and it would delay making payments for some activities, default on its debt obligations, or both."

Zandi says the X-Date is around Oct. 20

However, others think there is much less time.

Mark Zandi, chief economist at Moody's Analytics, told CNN on Wednesday that Yellen is "likely being somewhat conservative," but not overly so.

"I wouldn't be surprised if it is a day or two later than the 18th," Zandi said. "I'm estimating the 20th."

That implies a bit of wiggle room, but not much, certainly not as much as in 2011. Zandi said Thursday he's not changing his estimate — despite the more optimistic take by the CBO.

Very hard to predict

The problem is that nailing down a precise X-date is unusually difficult today because there are even more moving pieces than during prior debt ceiling clashes.

Beyond the usual outlays for items like Social Security, Medicare and defense contracts, the Treasury is making big payments for disaster relief and Covid programs.

At the same time, tax revenue has been more volatile due to the impact of the pandemic.

"Usually, these things aren't very hard to predict. But right now, they are," said Thomas Simons, senior money market economist at Jefferies. "The variant in daily cash flows is so high relative to historical precedent."

Yellen noted the government's daily gross cash flow averaged nearly $50 billion per day over the past year and has even exceeded $300 billion.

'Scarily close to running out of cash'

Like Yellen, some on Wall Street are moving up the timing of when the government could default.

As recently as September 10, Jefferies projected that Treasury could avoid default until November 10-15, based on projections for cash balances and updated data on extraordinary measures.

"Since then, things have changed," Simons wrote in a note to clients on Tuesday.

In particular, Simons noted that Treasury's cash position "has dwindled faster than we expected it would throughout September."

That's never a good sign heading into a debt ceiling impasse.

Simons said Treasury's cash balances are starting to get "very low" on October 18. "We are more or less in agreement with Yellen," Simons wrote in a report titled "Yellen flags October 18...and we think she means it this time."

Asked on Thursday about the CBO report, Simons conceded there is a great deal of uncertainty and said the CBO forecast gives him a bit more hope — but not much.

"I still think that during the middle of October, Treasury is going to get scarily close to running out of cash," Simons told CNN. "I am still extremely concerned about what is going to happen in the middle of next month."