Gregorian Calendar

The Gregorian Calendar is named after Pope Gregory XIII who introduced it in 1582 CE.
It replaced the Julian Calendar introduced by Julius Caesar in 45 BCE.

The Julian Calendar added an extra day every four years, but this was too frequent.
It did not match the time taken for the Earth to circle the Sun known as the Tropical Year.

The Julian Calendar and the Gregorian Calendar have exactly the same number of months and the same number of days in each month.
In the Julian Calendar, one year consists of 365.25 days, with an extra day inserted every four years, called a Leap Year.
In the Gregorian Calendar, one year consists of 365.2425 days with an extra day inserted every four years, called a Leap Year.
But in the Gregorian Calendar, only years divisible by four can be Leap Years, and only century years divisible by 400 can be Leap Years.
Consequently, the Julian Calendar is currently 13 days behind the Gregorian Calendar until 2100, when it will be 14 days behind.
To convert from the Julian Calendar to the Gregorian Calendar, add 13 days.
To convert from the Gregorian Calendar to the Julian Calendar, subtract 13 days.

In the Julian Calendar, a year consists of 365.25 days, with an extra day inserted every four years, called a Leap Year. This is then repeated indefinitely. This makes it 10.8 minutes longer than the Gregorian Calendar.
The Julian Calendar gains about 3 days every 400 years with respect to the Solar Tropical Year of 365.24217 days.
This makes the Julian Calendar currently 13 days behind the Gregorian Calendar.
The Gregorian Calendar still gains 1 day, but only every 3,030 years.