（1）本代码来自于头文件 qcalendar . h ：
cpp
#ifndef QCALENDAR_H
#define QCALENDAR_H
#include <limits>
#include <QtCore/qglobal.h>
#include <QtCore/qlocale.h>
#include <QtCore/qstring.h>
#include <QtCore/qstringview.h>
/* Suggested enum names for other calendars known to CLDR (v33.1)
Not yet implemented - see QCalendar::System - contributions welcome:
* Buddhist -- Thai Buddhist, to be specific
* Chinese
* Coptic
* Dangi -- Korean
* Ethiopic (Amete Mihret - epoch approx. 8 C.E.)
* EthiopicAmeteAlem (Amete Alem - epoch approx. 5493 B.C.E; data from
type="ethiopic-amete-alem", an alias for type="ethioaa")
* Hebrew
* Indian -- National
* Islamic -- Based on astronomical observations, not predictions, so hard to
implement. CLDR's data for type="islamic" apply, unless overridden, to the
other Islamic calendar variants, i.e. IslamicCivil, above, and the three
following. See QHijriCalendar, a common base to provide that data.
* IslamicTabular -- tabular, astronomical epoch (same as IslamicCivil, except
for epoch), CLDR type="islamic-tbla"
* Saudi -- Saudi Arabia, sighting; CLDR type="islamic-rgsa"
* UmmAlQura -- Umm al-Qura, Saudi Arabia, calculated; CLDR type="islamic-umalqura"
* Iso8601 -- as Gregorian, but treating ISO 8601 weeks as "months"
* Japanese -- Imperial calendar
* Minguo -- Republic of China, Taiwan; CLDR type="roc"
See:
http://www.unicode.org/repos/cldr/tags/latest/common/bcp47/calendar.xml
These can potentially be supported, as features, using CLDR's data; any
others shall need hand-crafted localization data; it would probably be best
to do that by contributing data for them to CLDR.
*/
QT_BEGIN_NAMESPACE
class QCalendarBackend;
class QDate;
/*
The QCalendar class describes calendar systems.
QCalendar对象使用特定系统的规则将年、月和日数映射到特定的日期(最终由其儒略日数标识).
默认的 QCalendar()是一个推算的格里高利历，没有零年。
通过启用合适的功能或加载插件，可以支持其他日历。
作为功能支持的日历可以通过将QCalendar:System 枚举传递给构造函数来构建。
所有支持过的日历在构建后都可以通过名称来构建。
(因此插件会实例化它们的日历后端以进行注册。)
通过 QCalendar::System 可访问的内置后端也总是可以通过名称获取。
使用自定义后端的时间表也可以在构建时使用分配给后端的-个唯-ID来构建。
QCalendar值是不可变的。
*/
class Q_CORE_EXPORT QCalendar
{
Q_GADGET
private:
// Always supplied by QCalendarBackend and expected to be a singleton
// Note that the calendar registry destroys all backends when it is itself
// destroyed. The code should check if the registry is destroyed before
// dereferencing this pointer.
const QCalendarBackend * d_ptr;
public:
// (Extra parentheses to suppress bogus reading of min() as a macro.)
enum : int { Unspecified = (std::numeric_limits<int>::min)() };
struct YearMonthDay
{
YearMonthDay() = default;
YearMonthDay(int y, int m = 1, int d = 1) : year(y), month(m), day(d) {}
bool isValid() const
{ return month != Unspecified && day != Unspecified; }
// (The first year supported by QDate has year == Unspecified.)
int year = Unspecified;
int month = Unspecified;
int day = Unspecified;
};
// Feature (\w+)calendar uses CLDR type="\1" data,
// except as noted in type="..." comments below
enum class System
{
Gregorian, // CLDR: type = "gregory", alias = "gregorian"
#ifndef QT_BOOTSTRAPPED
Julian = 8,
Milankovic = 9,
#endif // These are Roman-based, so share Gregorian's CLDR data
// Feature-controlled calendars:
#if QT_CONFIG(jalalicalendar) // type="persian"
Jalali = 10,
#endif
// type="islamic-civil", uses data from type="islamic"
#if QT_CONFIG(islamiccivilcalendar)
IslamicCivil = 11,
// tabular, civil epoch
// 30 year cycle, leap on 2, 5, 7, 10, 13, 16, 18, 21, 24, 26 and 29
// (Other variants: 2, 5, 8, (10|11), 13, 16, 19, 21, 24, 27 and 29.)
#endif
Last = 11, // Highest number of any above
User = -1
};
// New entries must be added to the \enum doc in qcalendar.cpp and
// handled in QCalendarBackend::fromEnum()
Q_ENUM(System) //接入 Qt 的元对象系统
class SystemId
{
size_t id;
friend class QCalendarBackend;
constexpr bool isInEnum() const
{ return id <= size_t(QCalendar::System::Last); }
constexpr explicit SystemId(QCalendar::System e) : id(size_t(e)) { }
constexpr explicit SystemId(size_t i) : id(i) { }
public:
constexpr SystemId() : id(~size_t(0)) {}
constexpr size_t index() const noexcept { return id; }
constexpr bool isValid() const noexcept { return ~id; }
};
explicit QCalendar(); // Gregorian, optimised
explicit QCalendar(System system);
// ### Qt 7: remove
explicit QCalendar(QLatin1String name);
// ### Qt 7: use QAnyStringView
explicit QCalendar(QStringView name);
explicit QCalendar(SystemId id);
// QCalendar is a trivially copyable value type.
bool isValid() const { return d_ptr != nullptr; }
// Date queries:
int daysInMonth(int month, int year = Unspecified) const;
int daysInYear(int year) const;
int monthsInYear(int year) const;
bool isDateValid(int year, int month, int day) const;
// Leap years:
bool isLeapYear(int year) const;
// Properties of the calendar:
bool isGregorian() const;
bool isLunar () const;
bool isLuniSolar() const;
bool isSolar () const;
bool isProleptic() const;
bool hasYearZero() const;
int maximumDaysInMonth () const;
int minimumDaysInMonth () const;
int maximumMonthsInYear() const;
QString name() const;
// QDate conversions:
QDate dateFromParts(int year, int month, int day) const;
QDate dateFromParts(const YearMonthDay & parts) const;
YearMonthDay partsFromDate(QDate date) const;
int dayOfWeek(QDate date) const;
// Month and week-day names (as in QLocale):
QString monthName( const QLocale & locale,
int month, int year = Unspecified ,
QLocale::FormatType format = QLocale::LongFormat) const;
QString standaloneMonthName(const QLocale & locale,
int month, int year = Unspecified ,
QLocale::FormatType format = QLocale::LongFormat) const;
QString weekDayName( const QLocale & locale, int day,
QLocale::FormatType format = QLocale::LongFormat) const;
QString standaloneWeekDayName(const QLocale & locale, int day,
QLocale::FormatType format = QLocale::LongFormat) const;
// Formatting of date-times:
QString dateTimeToString(QStringView format, const QDateTime & datetime,
QDate dateOnly, QTime timeOnly,
const QLocale & locale) const;
// What's available ?
static QStringList availableCalendars();
}; //完结 class Q_CORE_EXPORT QCalendar
QT_END_NAMESPACE
#endif // QCALENDAR_H（2）儒略历，格里高利历原来就是公历 ：
（3）现代公历 ：
（4）
谢谢