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.
Starting Date Range of Gregorian Calendar | Conversion Used | Resulting Date Range of Julian-1 Calendar | Notes |
---|---|---|---|
March 1, 2100 to February 28, 2200 | Count back 14 days | February 16, 2100 to February 14, 2200 | No Feb 29, 2200 Gregorian |
March 1, 1900 to February 28, 2100 | Count back 13 days | February 17, 1900 to February 15, 2100 | No Feb 29, 2100 Gregorian |
March 1, 1800 to February 28, 1900 | Count back 12 days | February 18, 1800 to February 16, 1900 | No Feb 29, 1900 Gregorian |
March 1, 1700 to February 28, 1800 | Count back 11 days | February 19, 1700 to February 17, 1800 | No Feb 29, 1800 Gregorian |
March 1, 1500 to February 28, 1700 | Count back 10 days | February 20, 1500 to February 18, 1700 | No Feb 29, 1700 Gregorian |
March 1, 1400 to February 28, 1500 | Count back 9 days | February 21, 1400 to February 19, 1500 | No Feb 29, 1500 Gregorian |
March 1, 1300 to February 28, 1400 | Count back 8 days | February 22, 1300 to February 20, 1400 | No Feb 29, 1400 Gregorian |
March 1, 1100 to February 28, 1300 | Count back 7 days | February 23, 1100 to February 21, 1300 | No Feb 29, 1300 Gregorian |
March 1, 1000 to February 28, 1100 | Count back 6 days | February 24, 1000 to February 22, 1100 | No Feb 29, 1100 Gregorian |
March 1, 900 to February 28, 1000 | Count back 5 days | February 25, 900 to February 23, 1000 | No Feb 29, 1000 Gregorian |
March 1, 700 to February 28, 900 | Count back 4 days | February 26, 700 to February 24, 900 | No Feb 29, 900 Gregorian |
March 1, 600 to February 28, 700 | Count back 3 days | February 27, 600 to February 25, 700 | No Feb 29, 700 Gregorian |
March 1, 500 to February 28, 600 | Count back 2 days | February 28, 500 to February 26, 600 | No Feb 29, 600 Gregorian |
March 1, 300 to February 28, 500 | Count back 1 days | February 29, 300 to February 27, 500 | No Feb 29, 500 Gregorian |
March 1, 200 to February 28, 300 | Count 0 days | March 1, 200 to February 28, 300 | No Feb 29, 300 Gregorian |
March 1, 100 to February 28, 200 | Count ahead 1 days | March 2, 100 to February 29, 200 | No Feb 29, 200 Gregorian |
March 1, 101 BC to February 28, 100 AD | Count ahead 2 days | March 3, 101 BC to March 1, 100 AD | No Feb 29, 100 AD Gregorian |
March 1, 201 BC to February 28, 101 BC | Count ahead 3 days | March 4, 201 BC to March 2, 101 BC | No Feb 29, 101 BC Gregorian |
March 1, 301 BC to February 28, 201 BC | Count ahead 4 days | March 5, 301 BC to March 3, 201 BC | No Feb 29, 201 BC Gregorian |
March 1, 501 BC to February 28, 301 BC | Count ahead 5 days | March 6, 501 BC to March 4, 301 BC | No Feb 29, 301 BC Gregorian |
March 1, 601 BC to February 28, 501 BC | Count ahead 6 days | March 7, 601 BC to March 5, 501 BC | No Feb 29, 501 BC Gregorian |
Starting Gregorian Date | Conversion Used | Resulting Julian-1 Date |
---|---|---|
February 24, 1900 | Back 12 days | February 12, 1900 |
February 25, 1900 | Back 12 days | February 13, 1900 |
February 26, 1900 | Back 12 days | February 14, 1900 |
February 27, 1900 | Back 12 days | February 15, 1900 |
February 28, 1900 | Back 12 days | February 16, 1900 |
February 29, 1900 | Not Applicable, No Such Date | 1900 is not a Gregorian leap year because it is divisible by 100 but not by 400. |
March 1, 1900 | Back 13 days | February 17, 1900 (use Feb 29 when counting back days in the Julian-1 calendar) |
March 2, 1900 | Back 13 days | February 18, 1900 |
March 3, 1900 | Back 13 days | February 19, 1900 |
March 4, 1900 | Back 13 days | February 20, 1900 |
March 5, 1900 | Back 13 days | February 21, 1900 |
March 6, 1900 | Back 13 days | February 22, 1900 |
March 7, 1900 | Back 13 days | February 23, 1900 |
March 8, 1900 | Back 13 days | February 24, 1900 |
March 9, 1900 | Back 13 days | February 25, 1900 |
March 10, 1900 | Back 13 days | February 26, 1900 |
March 11, 1900 | Back 13 days | February 27, 1900 |
March 12, 1900 | Back 13 days | February 28, 1900 |
March 13, 1900 | Back 13 days | February 29, 1900 |
March 14, 1900 | Back 13 days | March 1, 1900 |
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