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Shift the base date for the Gregorian calendar's calculations

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Internally it was using the start of March 4801 BCE and then adjusting
the year by 4800; instead, use the start of March 1 BCE and save those
adjustments. This incidentally aligns better with the Milankovic
calendar's calculations.

Change-Id: Ic64b4e1d4f65bbf1973a77f830e68f05f7949f4d
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
This commit is contained in:
Edward Welbourne 2023-01-23 20:05:57 +01:00
parent 5a91b734f2
commit 0e653246b1
1 changed files with 8 additions and 4 deletions
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12
src/corelib/time/qgregoriancalendar.cpp
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@ -162,6 +162,10 @@ int QGregorianCalendar::yearSharingWeekDays(QDate date)
* Math from The Calendar FAQ at http://www.tondering.dk/claus/cal/julperiod.php
* This formula is correct for all julian days, when using mathematical integer
* division (round to negative infinity), not c++11 integer division (round to zero).
*
* The source given uses 4801 BCE as base date; the following adjusts that by
* 4800 years to simplify part of the arithmetic (and match more closely what we
* do for Milankovic).
*/
bool QGregorianCalendar::julianFromParts(int year, int month, int day, qint64 *jd)
@ -174,16 +178,16 @@ bool QGregorianCalendar::julianFromParts(int year, int month, int day, qint64 *j
++year;
int a = month < 3 ? 1 : 0;
qint64 y = qint64(year) + 4800 - a;
qint64 y = qint64(year) - a;
int m = month + 12 * a - 3;
*jd = day + qDiv<5>(153 * m + 2) - 32045
*jd = day + qDiv<5>(153 * m + 2) + 1721119
+ 365 * y + qDiv<4>(y) - qDiv<100>(y) + qDiv<400>(y);
return true;
}
QCalendar::YearMonthDay QGregorianCalendar::partsFromJulian(qint64 jd)
{
qint64 a = jd + 32044;
qint64 a = jd - 1721120;
qint64 b = qDiv<146097>(4 * a + 3);
int c = a - qDiv<4>(146097 * b);
@ -191,7 +195,7 @@ QCalendar::YearMonthDay QGregorianCalendar::partsFromJulian(qint64 jd)
int e = c - qDiv<4>(1461 * d);
int m = qDiv<153>(5 * e + 2);
int y = 100 * b + d - 4800 + qDiv<10>(m);
int y = 100 * b + d + qDiv<10>(m);
// Adjust for no year 0
int year = y > 0 ? y : y - 1;