In the year 1752, British and (soon-to-be) American citizens went to bed on the night of September 2nd...and all woke up on September 14th. How did they lose 11 days?
From Roman times, the Western world had worked with a calendar developed by Julius Caesar - the 'Julian Calendar'. But this calendar wasn't perfectly accurate, with its years actually being around 11 minutes out. Which isn't so bad over a few years, but after a millennium or two adds up to days.
By 1582 the mistake had multiplied out to 10 days, and the Catholic Church didn't like what it was doing to the timing of Easter. So Pope Gregory XIII instituted a new calendar - the one we use today (the Gregorian calendar) - but to do so the error needed to be corrected. Pope Gregory therefore decreed that that October 4th 1582 would become October 15th.
Protestants weren't such big fans of the Catholics, so they weren't about to listen to any papal decrees. It took them another 170 years to start instituting the change to the Gregorian calendar, with Britain and its colonies (including the soon to be United States) not changing over until 1752.
By that time, the error in the Julian calendar had built up to 11 days - so to fix the problem, in 1752 September 2nd was immediately followed by September 14th. Children with birth certificates showing each of those two dates were only born a day apart.
And if that isn't confusing enough: at that time the year began on March 25th, but with the calendar revision this was shifted to January 1 - so the year 1752 ran from March 25 to December 31, with September only being 19 days long as well.
This change in calendar results in some odd historical quirks, such as George Washington's birth certificate showing that he was born on February 11, 1731, while his birthday is now celebrated by us on February 22nd, 1732.
An important historical note to remember when setting the dates on your DeLorean!