If you input the calendar date/time of 1582-10-04 00:00 or earlier, the resulting Julian day number will be with respect to the Julian calendar. Any date/time after 1582-10-04 00:00 will result in a Julian day number with respect to the Gregorian proleptic calendar.
This calendar was named the Julian calendar, after Julius Caesar, and it continues to be used by Eastern Orthodox churches for holiday calculations to this day. However, despite the correction, the Julian calendar is still 11 1 /2 minutes longer than the actual solar year, and after a number of centuries, even 11 1 /2 minutes adds up.
In 45 B.C., New Year’s Day is celebrated on January 1 for the first time in history as the Julian calendar takes effect. Soon after becoming Roman dictator, Julius Caesar decided that the

In astronomy, a Julian year (symbol: a) is a unit to measure time. It has exactly 365 1 ⁄ 4 days of 86,400 seconds each. That is the average length of the year in the Julian calendar used in Western societies in previous centuries, and for which the unit is named.

A year in the Ethiopian calendar is 13 months long, with 12 months of 30 days each. The last month has 5 days in a common year and 6 days during leap year. Like in the Julian calendar, a leap year in the Ethiopian calendar happens every 4 years without exception.
The approximation 365 1/4 is achieved by having 1 leap year every 4 years (as explained above) and the rule for calculation is that every year divisible by 4 is a leap year. However, this rule was not followed in the first years after the introduction of the Julian calendar in 45 BC.
The Julian calendar has 365.25 days while the Gregorian calendar has 365.2425 days. Because of the gradual shift in the orientation of Earth's axis of rotation, phenomenon discovered by the Greek astronomer Hipparchus of Nicaea and known as the axial precession, the solar year (also known as the tropical year) is about 20 minutes shorter than
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  • year in julian calendar