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Conversion Between Julian-1 and Gregorian Calendars

Converting dates between the Julian-1 and Gregorian calendars can get tricky due to the confusing rules of leap year dates that differ between the two calendars.

Note that in the Julian-1 and proleptic Julian-1 calendars, years after 1 AD are evenly divisible by four are leap years. Examples are 4, 8, 12, 16, etc. In the years prior to 1 AD, and remember that there is no year zero in the calendar, if you add one year to a BC year and it can be divided evenly by four, then it's a leap year. 1 BC + 1 = 0, which is a leap year. 2 BC + 1 = -1, which is not a leap year. 3 BC + 1 = -2 and is not a leap year. 4 BC is not a leap year because 4 BC + 1 = 3, which is not divisible by four. 5 BC is a leap year since 5 BC + 1 = -4.

In the Gregorian calendar, the rules of leap years are more complex. A Gregorian year is a leap year on every year after 1 AD that is evenly divisible by four, except for the years that are divisible by 100 but not in turn divisible by 400. For example: 1600 and 2000 were leap years. 1700, 1800, 1900 were not leap years. 2100, 2200, 2300 are not going to be leap years. 2400 will be a leap year. In the years prior to 1 AD, and again, the Gregorian calendar has no year zero, you need to add one year to a BC year to get a chronological math number that can be easily used. Convert a BC year to a math number by adding 1 year to it. Examples: 1 BC + 1 = 0, which is divisible by 4 and 400. 2 BC + 1 = -1, which is not divisible by four and is not a leap year. 5 BC + 1 = -4, which is divisible by four and is a leap year. 101 BC + 1 = -100 is divisible by 100 but not by 400, so therefore, it's not a leap year. 401 BC + 1 = -400 is a leap year since it's divisible by 400.

When converting from Gregorian to Julian-1 dates, first, understand that you use February 29 on all years divisible by four (or BC numbers converted to a math number) to count backwards to a date. So, to convert March 1, 2100 Gregorian to a Julian-1 date, you count back 14 days beginning on this March 1 date, with March 1 counted as a count of zero, then February 29 is counted one, February 28 is counted two, February 27 is counted three, February 26 is counted four, and so forth until you count to 14 and reach February 16, 2100 Julian-1 date. From March 1, 2100 Gregorian date to February 28, 2200, you go back 14 days from the date to get to the Julian-1 date, and remember, there is a February 29 every four years on the Julian-1 calendar.

Here's a chart I created to help out.

Gregorian to Julian-1 Calendar Date Converter

Starting Date Range of Gregorian CalendarConversion UsedResulting Date Range of Julian-1 CalendarNotes
March 1, 2100 to February 28, 2200Count back 14 daysFebruary 16, 2100 to February 14, 2200No Feb 29, 2200 Gregorian
March 1, 1900 to February 28, 2100Count back 13 daysFebruary 17, 1900 to February 15, 2100No Feb 29, 2100 Gregorian
March 1, 1800 to February 28, 1900Count back 12 daysFebruary 18, 1800 to February 16, 1900No Feb 29, 1900 Gregorian
March 1, 1700 to February 28, 1800Count back 11 daysFebruary 19, 1700 to February 17, 1800No Feb 29, 1800 Gregorian
March 1, 1500 to February 28, 1700Count back 10 daysFebruary 20, 1500 to February 18, 1700No Feb 29, 1700 Gregorian
March 1, 1400 to February 28, 1500Count back 9 daysFebruary 21, 1400 to February 19, 1500No Feb 29, 1500 Gregorian
March 1, 1300 to February 28, 1400Count back 8 daysFebruary 22, 1300 to February 20, 1400No Feb 29, 1400 Gregorian
March 1, 1100 to February 28, 1300Count back 7 daysFebruary 23, 1100 to February 21, 1300No Feb 29, 1300 Gregorian
March 1, 1000 to February 28, 1100Count back 6 daysFebruary 24, 1000 to February 22, 1100No Feb 29, 1100 Gregorian
March 1, 900 to February 28, 1000Count back 5 daysFebruary 25, 900 to February 23, 1000No Feb 29, 1000 Gregorian
March 1, 700 to February 28, 900Count back 4 daysFebruary 26, 700 to February 24, 900No Feb 29, 900 Gregorian
March 1, 600 to February 28, 700Count back 3 daysFebruary 27, 600 to February 25, 700No Feb 29, 700 Gregorian
March 1, 500 to February 28, 600Count back 2 daysFebruary 28, 500 to February 26, 600No Feb 29, 600 Gregorian
March 1, 300 to February 28, 500Count back 1 daysFebruary 29, 300 to February 27, 500No Feb 29, 500 Gregorian
March 1, 200 to February 28, 300Count 0 daysMarch 1, 200 to February 28, 300No Feb 29, 300 Gregorian
March 1, 100 to February 28, 200Count ahead 1 daysMarch 2, 100 to February 29, 200No Feb 29, 200 Gregorian
March 1, 101 BC to February 28, 100 ADCount ahead 2 daysMarch 3, 101 BC to March 1, 100 ADNo Feb 29, 100 AD Gregorian
March 1, 201 BC to February 28, 101 BCCount ahead 3 daysMarch 4, 201 BC to March 2, 101 BCNo Feb 29, 101 BC Gregorian
March 1, 301 BC to February 28, 201 BCCount ahead 4 daysMarch 5, 301 BC to March 3, 201 BCNo Feb 29, 201 BC Gregorian
March 1, 501 BC to February 28, 301 BCCount ahead 5 daysMarch 6, 501 BC to March 4, 301 BCNo Feb 29, 301 BC Gregorian
March 1, 601 BC to February 28, 501 BCCount ahead 6 daysMarch 7, 601 BC to March 5, 501 BCNo Feb 29, 501 BC Gregorian

Sample Date Range for Gregorian to Julian-1 Calendar Date Converter

Starting Gregorian DateConversion UsedResulting Julian-1 Date
February 24, 1900Back 12 daysFebruary 12, 1900
February 25, 1900Back 12 daysFebruary 13, 1900
February 26, 1900Back 12 daysFebruary 14, 1900
February 27, 1900Back 12 daysFebruary 15, 1900
February 28, 1900Back 12 daysFebruary 16, 1900
February 29, 1900Not Applicable, No Such Date1900 is not a Gregorian leap year because it is divisible by 100 but not by 400.
March 1, 1900Back 13 daysFebruary 17, 1900 (use Feb 29 when counting back days in the Julian-1 calendar)
March 2, 1900Back 13 daysFebruary 18, 1900
March 3, 1900Back 13 daysFebruary 19, 1900
March 4, 1900Back 13 daysFebruary 20, 1900
March 5, 1900Back 13 daysFebruary 21, 1900
March 6, 1900Back 13 daysFebruary 22, 1900
March 7, 1900Back 13 daysFebruary 23, 1900
March 8, 1900Back 13 daysFebruary 24, 1900
March 9, 1900Back 13 daysFebruary 25, 1900
March 10, 1900Back 13 daysFebruary 26, 1900
March 11, 1900Back 13 daysFebruary 27, 1900
March 12, 1900Back 13 daysFebruary 28, 1900
March 13, 1900Back 13 daysFebruary 29, 1900
March 14, 1900Back 13 daysMarch 1, 1900
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