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No guidance is provided about conversion of dates before March 5, -500, or after February 29, 2100 (both being Julian dates). For unlisted dates, find the date in the table closest to, but earlier than, the date to be converted. Be sure to use the correct column. If converting from Julian to Gregorian, add the
The formulae can be used proleptically, but "Year 0" is in fact year 1 BC (see astronomical year numbering). The Julian calendar is in fact proleptic right up to 1 March AD 4 owing to mismanagement in Rome (but not Egypt) in the period since the calendar was put into effect on 1 January 45 BC (which was not a leap year).
A calendrical calculation is a calculation concerning calendar dates. Calendrical calculations can be considered an area of applied mathematics. Some examples of calendrical calculations: Converting a Julian or Gregorian calendar date to its Julian day number and vice versa (see § Julian day number calculation within that article for details).
The "basic" format for year 0 is the four-digit form 0000, which equals the historical year 1 BC. Several "expanded" formats are possible: −0000 and +0000, as well as five- and six-digit versions. Earlier years are also negative four-, five- or six-digit years, which have an absolute value one less than the equivalent BC year, hence -0001 = 2 BC.
The Julian date (JD) of any instant is the Julian day number plus the fraction of a day since the preceding noon in Universal Time. Julian dates are expressed as a Julian day number with a decimal fraction added. [8] For example, the Julian Date for 00:30:00.0 UT January 1, 2013, is 2 456 293.520 833. [9]
Subtracting that from 317 remainder days is 307; in other words, the 307th day of the year 644 CE, which is November 3. To summarize: the Long Count date 9.10.11.17.0 corresponds to November 3, 644 CE, in the Proleptic Gregorian calendar. To convert a Julian day to a Julian/Gregorian astronomical date (Proleptic Julian calendar before 46 BCE):
year Remain-der on divide by 900 Is a Revised Julian leap year Is a Grego-rian leap year Revised Julian is same as Grego-rian 1000: 100 1100: 200 1200: 300 1300: 400 1400: 500 1500: 600 1600: 700 1700 800 1800 0 1900 100 2000 200 2100 300 2200 400 2300 500 2400 600 2500 700 2600 800 2700 0 2800: 100 2900: 200 3000: 300 3100: 400 3200: 500 3300: ...
Simple formulae for Julian day numbers and calendar dates. Quarterly Journal of the Royal Astronomical Society, 25, 53. ... Text is available under the Creative ...