The U.S. Department of the Treasury, Bureau of the Public Debt on its TreasuryDirect website, Debt to the Penny section, publishes - every business day by 3 PM - the Public Debt amount that was outstanding at the end of the previous business day. The system relies on reporting entities (for example, Federal Reserve Banks) that report a variety of Treasury security information at the end of each business day.
Worldometer has developed an algorithm which calculates the current estimated rate of change of the amount of debt outstanding in between the daily US Treasury updates. The formula components are recalculated daily as the latest official US National Debt data is published, so that the algorithm continuously adjusts itself accordingly. The magnitude and even direction of the daily changes can be erratic and unpredictable. For example, even though on a yearly or monthly basis the US debt might be overall increasing (as it is the case now), there are days when the US debt will decrease with respect to the previous day, only because on that particular day there happened to be more redemptions of Treasury securities than there were issues. During these limited time intervals, unless the change in direction and magnitude is in any way representative of a trend, the US debt clock will not show a decreasing debt, as it clearly would not provide an accurate representation of the current trend, but it will instead show the debt increasing at the current average rate, and eventually re-adjusting suddenly to a lower level.
Simplifying, every year the United States Government collects revenue from taxes and spends it on its public programs and agencies. If in any given year it spends more than it earns, its yearly budget will result in a deficit. To cover the deficit, the U.S. Treasury will have to borrow money (by selling securities like Treasury bills, notes, bonds and savings bonds to the public), just like an individual who spends more than what he earns will have to borrow the missing amount from a credit card. The accumulated deficits year after year form the outstanding US national debt.
The current U.S. debt is $23,008 trillions as of Oct 31, 2019.