and Wolf were discussing the neighboring states when the former VP mixed up the dates surrounding when became independent.
"I want to remind you that Delaware used to be part of Pennsylvania," Wolf told Biden during the livestream.
"That's right, but we declared our independence on December the 7, by the way," Biden said.
"It's not just D-Day," he added.
D-Day – the beginning of the operation to free north-west Europe from the Nazis – was actually on June 6, 1944.
December 7 is clouded by the Pearl Harbor attacks, when Japanese forces launched a surprise bombing on the US naval base in Hawaii in 1941.
More than 2,400 Americans were killed.
Delaware officially declared its independence on June 15, 1776.
The blunder was first pointed out on by Zach Parkinson, deputy director of communications and researcher for President Donald Trump.
December 7 was an important date for Delaware, as it was the day the state was the first to ratify the federal constitution in 1787.
The date even appears on the state flag – but it was not the day the state declared independence.
Biden has called himself a "gaffe machine" – and President Trump has dubbed him "Sleepy Joe".
The mix-up came just more than a week after .
Biden claimed 85,000 jobs in America had been lost as a result of , and millions of Americans had died.
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"We're ... in the middle of a pandemic that has cost us more than 85,000 jobs as of today. Lives of millions of people. Millions of people. Millions of jobs," the Democratic candidate said during a virtual roundtable on PBS.
The numbers are actually opposite.
At the time, at least 85,000 people in the US had died from coronavirus, while 36.5million lost their jobs.