(12C) Gregorian Date to Julian Day Number and vice versa

[Albert Chan]

From HP-12C owner's manual, valid date range = Oct 15, 1582 to Nov 25, 4046

It is trivial to extend the range, using the fact Gregorian calendar cycle of 400 years.
Average days per year = (400*365 + 97 leap years)/400= 146097/400 = 365.2425

Example:

CJDN(Dec 31, 9999)
= CJDN(Dec 31, 1999) + 8000 years
= (2451545-1) + 8000 * 365.2425
= 5373484

Since 146097 % 7 = 0, weekday(Dec 31, 9999) = weekday(Dec 31, 1999) = Friday