Julian Calendar

The Julian Calendar was introduced by Julius Caesar in 45 BCE and was a reform of the old Roman Calendar.
It continued in use until 1582 CE, when it was replaced by the Gregorian Calendar.

The Julian Year consisted of 365.25 days, with an extra day inserted every three years, called a Leap Year. Eventually, the extra day was inserted every four years under Augustus.
Julius Caesar was advised by the Astronomer Sosigenes at Alexandria to abandon the lunar year in favour of the solar (Tropical) year.
The Julian Calendar then remained in place until the introduction of the Gregorian Calendar in 1582 CE.

The Julian Calendar added an extra day every four years, called a Leap Year, but this was too frequent.
It did not match the time taken for the Earth to circle the Sun known as the Tropical Year, and became several days behind the Tropical Year.
The Gregorian Calendar, named after Pope Gregory XIII, replaced the Julian Calendar in 1582 CE and realigned the Calendar with the Solar Year.
The Gregorian Year consists of 365.2425 days, not 365.25 days, with an extra day inserted almost every four years, but with some exceptions (see below).

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 CE, 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 three 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.
Whereas, the Gregorian Calendar only gains 1 day every 3,030 years with respect to the Tropical Year.