They forgot to mention that in 1752 the calendar skipped 11 days
in countries ruled by England, which includes the USA!
It is widely known that in September 1752, Great Britain switched from the Julian Calendar to the Gregorian Calendar. In order to achieve the change, 11 days were 'omitted' from the calendar - i.e. the day after 2 September 1752 was 14 September 1752.
Earth's orbit of sun is reason for the leap
Leap years are necessary because - rather inconveniently for calendar makers - it takes the Earth almost 6 hours longer than an even 365 days to complete one full orbit around the sun.
To keep things in sync, the Roman astronomer Sosigenes suggested adding one whole day to the calendar every four years.
Julius Caesar liked the idea and decreed that, beginning in 45 B.C., the new and improved Julian calendar, incorporating the leap year, would go into effect.
But tiny inaccuracies in the system accrued and, by the 16th century, the civil and liturgical calendar was out of step with the astronomical calendar by some 10 days.
In 1582, Pope Gregory XIII modified the Julian calendar, basically by eliminating three leap years every four centuries.
Because it was a Catholic innovation, the Gregorian calendar was slow to be adopted in many Protestant and Orthodox countries. Britain (and its American colonies) didn't adjust its calendar until 1752; Greece didn't accept the new system until 1923, the last European country to do so.
The rule for leap years: Add an extra day at the end of February to every year that's evenly divisible by four. An exception occurs if the year is evenly divisible by 100, but there's an exception to the exception if the year is also evenly divisible by 400.
Eventually there may be an exception to the exception to the exception. But because that won't happen until the year 4000, there's no sense worrying about it now.