Is there a way to correctly count number of days in 1582

Question

How would one calculate a number of days in 1582. Yes, that is the year of introduction of the Georgian Calendar (in some countries). I assume October 1582 should not have 31 days as some of the dates never existed.

Yet when I tried Joda Time (Java/Groovy) it says 30 days:

LocalDate start = new LocalDate("1582-10-01");
LocalDate end = new LocalDate("1582-10-31");
println Days.daysBetween(start, end).getDays();

Same for SQL

-- PostgreSQL
SELECT DATE_PART('day', '1582-10-31'::date - '1582-10-01'::timestamp);
   
-- MSSQL
SELECT DATEDIFF(dd, '1582-10-31', '1582-10-01');

So is there some agreement/specification to actually treat 1582-10-14 as if it would actually exist? Or is there some easy way to calculate correct diff for year 1582 and earlier?

Answer 1

I think I will go ahead and close with that both answers are probably correct. October 1582 did and didn't have 31 days. I mean that 14th October didn't exist (as in no one was born on that day in Gregorian Calendar) and for the purpose of accounting all debts were pushed by ten days. So I guess the only way is to manually count days and don't use any libraries for that.

When establishing Gregorian Calendar it was said that:

we direct and ordain: that ten days shall be removed from the month of October of the year 1582

But also:

But in order that nobody suffers prejudice by this our subtraction of ten days, in connection with any annual or monthly payments, the judges in any controversies that may arise over this, shall by reason of the said subtraction add ten days to the due date for any such payment.

Source: https://en.wikisource.org/wiki/Translation:Inter_gravissimas

Answer 2

I have not used Java in many years, but I am familiar with dealing with several calendars in other languages. From the "Key Concepts" subtab of the "Documentation" tab of the Joda Time website we find the "Chronology" page which states

The default chronology in Joda-Time is ISO. This calendar system is the same as that used by business in the majority of the world today. The ISO system is unsuitable for historical work before 1583 as it applies the leap year rules from today back in time (it is a proleptic calendar). As a result, users requiring a more historically accurate calendar system are forced to think about their actual requirements, which we believe is a Good Thing.

Proleptic means that from a known day and date that virtually everyone agrees about, such as the Meter Convention having been signed in Paris on 20 May 1875, the rules of the calendar are applied backward to find any date desired, even if it is before the calendar was created.

As for computing the interval in one calendar, such as the Julian calendar, to a date in a different calendar, such as the Gregorian calendar, a common approach is to convert them both to a count-of-days from a chosen epoch, such as the modified julian date, which counts from midnight universal time at the beginning of November 17, 1858. Then one simply subtracts one day count from the other to find the number of days between them. A quick glance at the Joda Time documentation did not show any facility for computing a day count.

I am currently not set up to program in Java. Ole V.V. comment about using the Gregorian-Julian chronology of Joda-Time seems useful, but I have not tried it:

LocalDate first = new LocalDate(1582, 10, 1, GJChronology.getInstance());
LocalDate last = new LocalDate(1582, 10, 31, GJChronology.getInstance());
int countOfDaysDiff = Days.daysBetween(first, last).getDays();
System.out.println(countOfDaysDiff);

Output according to Ole V.V.:

20