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 "qromancalendar_data_p.h"
7#include "qcalendarmath_p.h"
8
9#include <QtCore/qdatetime.h>
10
11QT_BEGIN_NAMESPACE
12
13using namespace QRoundingDown;
14
15/*!
16 \since 5.14
17
18 \class QJulianCalendar
19 \inmodule QtCore
20 \brief The QJulianCalendar class provides Julian calendar system
21 implementation.
22
23 \section1 Julian Calendar
24
25 The Julian calendar, proposed by Julius Caesar in 46 BC (708 AUC), was a
26 reform of the Roman calendar. It took effect on 1 January 45 BC (AUC 709),
27 by edict. It was the predominant calendar in the Roman world, most of
28 Europe, and in European settlements in the Americas and elsewhere, until it
29 was refined and gradually replaced by the Gregorian calendar,
30 promulgated in 1582 by Pope Gregory XIII.
31
32 The Julian calendar gains against the mean tropical year at the rate of one
33 day in 128 years. For the Gregorian calendar, the figure is one day in
34 3030 years. The difference in the average length of the year
35 between Julian (365.25 days) and Gregorian (365.2425 days) is 0.002%.
36
37 Source: \l {https://en.wikipedia.org/wiki/Julian_calendar}{Wikipedia page on
38 Julian Calendar}
39 */
40
41QString QJulianCalendar::name() const
42{
43 return QStringLiteral("Julian");
44}
45
46QStringList QJulianCalendar::nameList()
47{
48 return { QStringLiteral("Julian") };
49}
50
51bool QJulianCalendar::isLeapYear(int year) const
52{
53 if (year == QCalendar::Unspecified || !year)
54 return false;
55
56 return qMod<4>(a: year < 0 ? year + 1 : year) == 0;
57}
58
59// Julian Day 0 was January the first in the proleptic Julian calendar's 4713 BC.
60using namespace QRomanCalendrical;
61// End a Julian four-year cycle on 1 BC's leap day (Gregorian Feb 27th):
62constexpr qint64 JulianBaseJd = LeapDayGregorian1Bce - 2;
63
64bool QJulianCalendar::dateToJulianDay(int year, int month, int day, qint64 *jd) const
65{
66 Q_ASSERT(jd);
67 if (!isDateValid(year, month, day))
68 return false;
69
70 const auto yearDays = yearMonthToYearDays(year, month);
71 *jd = qDiv<4>(a: FourYears * yearDays.year) + yearDays.days + day + JulianBaseJd;
72 return true;
73}
74
75QCalendar::YearMonthDay QJulianCalendar::julianDayToDate(qint64 jd) const
76{
77 const auto year4Day = qDivMod<FourYears>(a: 4 * (jd - JulianBaseJd) - 1);
78 // Its remainder changes by 4 per day, except at roughly yearly quotient steps.
79 const auto ymd = dayInYearToYmd(dayInYear: qDiv<4>(a: year4Day.remainder));
80 const int y = year4Day.quotient + ymd.year;
81 return QCalendar::YearMonthDay(y > 0 ? y : y - 1, ymd.month, ymd.day);
82}
83
84QT_END_NAMESPACE
85

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