The Julian calendar was a reform of the Roman calendar introduced by Julius Caesar in 46 BC. It took effect in 45 BC.
It was the predominant calendar in most of Europe, and in European settlements in the Americas and elsewhere, until it was superseded by the Gregorian calendar.
It repeats completely every 1461 days, filling 4 years.
In the Julian calendar, any positive year is a leap year if divisible by 4. (Negative years are leap years if the absolute value divided by 4 yields a remainder of 1.)
Days are considered to begin at midnight.
The proleptic Julian calendar is produced by extending the Julian calendar backwards to dates preceding AD 4.
In the Julian calendar the average year has a length of 365.25 days. compared to the actual solar tropical year of 365.24219878 days.
The difference of the length of the Julian calendar year from the length of the real solar year is thus 0.0078 days, or 11 minutes.
Therefore, the Julian calendar accumulates one day of error with respect to the solar year every 128 years.
As the centuries passed the Julian Calendar became increasingly inaccurate with respect to the seasons.
This was especially troubling to the Roman Catholic Church because it affected the determination of the date of Easter,
which, by the 16th Century, was well on the way to slipping into summer.
For this reason, the Gregorian reform was instituted in the 16th century.
Ten days were omitted from the calendar, and it was decreed that the day following (Thursday) October 4, 1582 (which is October 5, 1582, in the old calendar)
would thenceforth be known as (Friday) October 15, 1582.
New rules for leap years were instituted, as detailed below.
The Gregorian solar calendar, the calendar in current use in the Western World, is an arithmetical calendar.
It counts days as the basic unit of time, grouping them into years of 365 or 366 days;
and repeats completely every 146,097 days, which fill 400 years, and which also happens to be 20,871 seven-day weeks.
Of these 400 years, 303 (the "common years") have 365 days, and 97 (the leap years) have 366 days.
This gives an average calendar-year length of exactly 365.2425 days, or 365 days, 5 hours, 49 minutes and 12 seconds.
The Gregorian calendar differs from the Julian only in the determination of leap years,
adding a correction for years divisible by 100 (not leap years) and 400 (leap years) in the Julian calendar.
The difference of the length of the Gregorian calendar year from the length of the real solar year is about 0.0003 days, or 26 seconds.
This calendar accumulates one day of error with respect to the solar year about every 3300 years.
The Gregorian calendar did not exist before October 15, 1582:
Friday, 10/15/1582 (Gregorian) immediately followed Thursday, 10/4/1582 (Julian).
The proleptic Gregorian calendar is produced by extending the Gregorian calendar backward to dates preceding its official introduction.
The Julian Day Number (JDN) is the integer assigned to a whole solar day in the Julian day count starting from noon Greenwich Mean Time,
with Julian day number 0 assigned to the day starting at noon on January 1, 4713 BC proleptic Julian calendar. (November 24, 4714 BC in the proleptic Gregorian calendar.)
This is the most recent day in which the year began on a Sunday with a full moon.
10/4/1982 julian = 10/4/1582 Gregorian = JDN 2299160
10/5/1582 julian = 10/15/1582 Gregorian = JDN 2299161
12/19/1999 julian = 1/1/2000 Gregorian JDN = 2451545
The Julian date includes fractions of a day. It begins at noon Universal Time (UT)