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Julian calendar converter, including Julian date calculation and dating ab Urbe condita (after the founding of Rome)

Use this converter to calculate dates in the Julian and Gregorian calendars. You can also calculate the Julian day, also known as Julian date and JDN, as well as make calculations of the date after the founding of Rome (in Latin, ab Urbe condita).

As you open this page, it should set the current date of the Gregorian calendar automatically.





ab Urbe condita, a.U.c.
(after the founding of Rome)

Julian day (or JDN,
Julian Day Number)

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I. The Gregorian calendar, also known as the Christian calendar and the Western calendar, is internationally the most widely accepted and used civil calendar. The Gregorian calendar was introduced by Pope Gregory XIII in 1582 A.D. The length of the Gregorian year is 365.2425 days, whereas the Julian year's length was 365.25 days (0.0075 days, or 10 minutes 48 seconds per year longer). The difference seems to be small but in terms of a thousand year period it is significant [the following calculation is approximate and is given for understanding of the process]: (1582 − (− 45))×0.0075 = 1627 × 0.0075 ≈ 12 days of accumulated error. So the Gregorian calendar began by skipping 10 calendar days, to restore March 21st as the date of the vernal equinox.

The last day of the Julian calendar was Thursday, 4 October 1582 and this was followed by the 1st day of the Gregorian calendar, Friday, 15 October 1582; the cycle of weekdays was not affected.

II. The Julian calendar was a reform of the Roman calendar.

The ancient Roman calendar consisted of 10 months, the first one being March, or Martius in Latin. At the turn of the VII-th and VI-th centuries B.C. the new, updated version of the calendar was adopted by Romans from Etruria (or Tyrrhenia: a region of Central Italy, where the Etruscans were living), in that new calendar a year was divided into 12 months, and, in particular, December was followed by January and February.

The Julian calendar was introduced by Julius Caesar, it took effect on January 1st, 45 B.C. (which equates to the year 709 ab Urbe condita, i.e. after the founding of Rome). Caesar was aided in his reform by the Egyptian astronomer Sosigenes of Alexandria. The Julian calendar has a regular year of 365 days divided into 12 months, plus the leap day is added to February every 4 years. Hence, there was a four year long solar cycle lasting 365×3 + 366 = 1461 days, and the year on average was 365 1/4 days long. The length of the months in the Julian calendar was as follows: 1) 30 days in April, June, September and November; 2) 31 days in January, March, May, July, August, October and December, 3) February was 28 days long for three years in a row, then 29 days long during the leap year. Caesar set the beginning of the year on the 1st of January (instead of March, as it was before).

The Julian year was intended to approximate the tropical (in other words, solar) year. Although Greek astronomers had known, that the tropical year was a few minutes shorter than 365.25 days, the calendar did not compensate for this difference. As a result, the calendar year gained about 3 days every four centuries compared to observed equinox times and the seasons. As explained above, this discrepancy was corrected by the Gregorian calendar reform of 1582 A.D. The Gregorian calendar has the same set of months and month lengths as the Julian calendar, but it inserts leap days according to a different rule. Consequently, the Julian calendar is currently 13 days behind the Gregorian calendar; for example, January 1st in the Julian calendar is January 14th in the Gregorian calendar.

The Julian calendar was the predominant calendar in most of Europe, and in European settlements in the Americas and elsewhere, until it was replaced by the Gregorian calendar.

III. Calculation of years ab Urbe condita, or A.U.C.: after the founding of the City (Rome). The city of Rome is said to have been founded in 753 Before Christ. So today, on July 22nd, 2013 A.D., when I'm typing this, it is 753 + 2013 = 2766th year after the founding of Rome. For your ease of use, use this page to convert Roman numerals to Arabic numerals and back again.

IV. The Julian day is a continuous count of days since the beginning of the Julian Period, and is used primarily by astronomers. The Julian Day Number (JDN) is the integer (i.e. numeral without a fractional part) 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). For instance, today, as I'm writing this on July 22nd, 2013, the Julian day is equal to 2,456,496. The Julian Date (JD) at any instant is the Julian day number for the preceding noon plus the fraction of the day since that instant.

This page was published on July 22, 2013.

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