Proleptic Julian calendar

[lorrden]

This is in the source:

/// This JDN corresponds to January 1, 45 BCE in the Julian calendar.
public static let effectiveJulianDayNumber: JulianDayNumber = 1704987

However in wikipedia have this interesting statement:

The proleptic Julian calendar is produced by extending the Julian calendar backwards to dates preceding AD 8 when the quadrennial leap year stabilized.

So I suspect that the isProleptic for the Julian Calendar calculates this wrongly.

[sbooth]

In general proleptic dates refer to dates before the epoch but for the Julian calendar it seems to be slightly different.

I based that date on Chapter 15 of the Explanatory Supplement to the Astronomical Almanac, 3rd edition which states:

By −46, the Roman calendar had gone badly awry; the months no longer followed the lunations and the year had lost step with the cycle of seasons (see Michels 1978; Bickerman 1980). This state of affairs was reformed by Julius Caesar (107–44 b.c.), who took the advice of the Alexandrian astronomer, Sosigenes.

Caesar first inserted 90 days into the year −45 to bring the months of the Roman calendar back to their traditional places with respect to the seasons. This year has been called “the last year of confusion.” He next changed the length of some of the old months. Finally he made provision for an intercalary day to be inserted every four years. This intercalary day (see § 15.3.5) was inserted before VI Kal. Mar. and termed Bis VI Kal. Mar.; it fell between VI I Kal. Mar. (February 23) and VI Kal. Mar. (February 25th). Years in which this was done were termed bissextile years.

...

Today the Julian calendar continues to be used by chronologists. The Julian proleptic calendar is formed by applying the rules of the Julian calendar to times before Caesar’s reform. This provides a simple chronological system for correlating other calendars and serves as the basis for the Julian Day Numbers.

Interestingly a different wikipedia page than the one you referenced states:

Augustus corrected errors in the observance of leap years by omitting leap days until AD 8. Julian calendar dates before March AD 4 are proleptic, and do not necessarily match the dates actually observed in the Roman Empire.[1]

So it seems there isn't strict agreement on what constitutes a proleptic Julian calendar date.

[lorrden]

The last paragraph on p.595 is interesting and consistent with Wikipedia.

Following Caesar’s death, the Roman calendrical authorities misapplied the leap-year rule, with the result that the intercalary day was inserted every third, rather than every fourth, year. This error was rectified by Augustus in −8. The details of this correction are uncertain but it is likely that he decreed that there should be no intercalation in the years −7 to +6 and that there should be intercalary days (leap days) inserted every fourth year from then on (i.e., in years 8,12,16 . . . ).

That means that the Julian calendar as used now is only valid from March 1, CE 4. And the point of "proleptic" is that dates before are extrapolated and not inline with historical dates. So I'd argue that wikipedia is correct on this, with respect to what one would want to specify proleptic for.

[sbooth]

While the Julian calendar didn't run smoothly until Augustus' correction it was Caesar's reform that really ended the difficulties and misalignment between the Roman civic calendar and the solar calendar; 46 BCE was the last year of confusion and contained 445 days. Wikipedia makes a reasonable case for using 8 CE as the start (end?) of the proleptic Julian calendar but it lacks any references for the choice. Richards is clear on how the proleptic Julian calendar is formed, and while not necessarily the final word, I haven't yet found any other published works to the contrary. If one uses the definition of extrapolated to times before its adoption for proleptic then the date of Caesar's reform seems to be appropriate.

It might be a good idea to add comments in the source covering some of this, and possibly a constant for the date of stabilization. I will keep looking for other books/articles on the topic to see if there are other accepted definitions of the proleptic Julian calendar. Mapping Time is written by Richards so agrees with Chapter 15 while Calendrical Calculations doesn't seem to directly address the issue.