1// Copyright (C) 2021 The Qt Company Ltd.
2// SPDX-License-Identifier: LicenseRef-Qt-Commercial OR LGPL-3.0-only OR GPL-2.0-only OR GPL-3.0-only
3
4#include "qglobal.h"
5#include "qjuliancalendar_p.h"
6#include "qcalendarmath_p.h"
7
8#include <QtCore/qdatetime.h>
9
10QT_BEGIN_NAMESPACE
11
12using namespace QRoundingDown;
13
14/*!
15 \since 5.14
16
17 \class QJulianCalendar
18 \inmodule QtCore
19 \brief The QJulianCalendar class provides Julian calendar system
20 implementation.
21
22 \section1 Julian Calendar
23
24 The Julian calendar, proposed by Julius Caesar in 46 BC (708 AUC), was a
25 reform of the Roman calendar. It took effect on 1 January 45 BC (AUC 709),
26 by edict. It was the predominant calendar in the Roman world, most of
27 Europe, and in European settlements in the Americas and elsewhere, until it
28 was refined and gradually replaced by the Gregorian calendar,
29 promulgated in 1582 by Pope Gregory XIII.
30
31 The Julian calendar gains against the mean tropical year at the rate of one
32 day in 128 years. For the Gregorian calendar, the figure is one day in
33 3030 years. The difference in the average length of the year
34 between Julian (365.25 days) and Gregorian (365.2425 days) is 0.002%.
35
36 Source: \l {https://en.wikipedia.org/wiki/Julian_calendar}{Wikipedia page on
37 Julian Calendar}
38 */
39
40QString QJulianCalendar::name() const
41{
42 return QStringLiteral("Julian");
43}
44
45QStringList QJulianCalendar::nameList()
46{
47 return { QStringLiteral("Julian") };
48}
49
50bool QJulianCalendar::isLeapYear(int year) const
51{
52 if (year == QCalendar::Unspecified || !year)
53 return false;
54
55 return qMod<4>(a: year < 0 ? year + 1 : year) == 0;
56}
57
58// Julian Day 0 was January the first in the proleptic Julian calendar's 4713 BC.
59using namespace QRomanCalendrical;
60// End a Julian four-year cycle on 1 BC's leap day (Gregorian Feb 27th):
61constexpr qint64 JulianBaseJd = LeapDayGregorian1Bce - 2;
62
63bool QJulianCalendar::dateToJulianDay(int year, int month, int day, qint64 *jd) const
64{
65 Q_ASSERT(jd);
66 if (!isDateValid(year, month, day))
67 return false;
68
69 const auto yearDays = yearMonthToYearDays(year, month);
70 *jd = qDiv<4>(a: FourYears * yearDays.year) + yearDays.days + day + JulianBaseJd;
71 return true;
72}
73
74QCalendar::YearMonthDay QJulianCalendar::julianDayToDate(qint64 jd) const
75{
76 const auto year4Day = qDivMod<FourYears>(a: 4 * (jd - JulianBaseJd) - 1);
77 // Its remainder changes by 4 per day, except at roughly yearly quotient steps.
78 const auto ymd = dayInYearToYmd(dayInYear: qDiv<4>(a: year4Day.remainder));
79 const int y = year4Day.quotient + ymd.year;
80 return QCalendar::YearMonthDay(y > 0 ? y : y - 1, ymd.month, ymd.day);
81}
82
83QT_END_NAMESPACE
84

source code of qtbase/src/corelib/time/qjuliancalendar.cpp