[Development] Calendar Systems proposal

[Soroush Rabiei]

Greetings all, and happy new year


> Issue: Q(Date|Time)+ think day-changes happen at midnight.  Some
>
calendar systems think they happen at sunset or sunrise; these are both
> rather tricky, as their time depends on date and latitude [3] - and I'm
> not sure what they do about days when the sun doesn't set or rise at
> all.  I don't see any hint of coping with this in KCalendarSystem and
> I'm fairly sure I don't want to solve that problem, so I think we stick
> with midnight and leave apps using the result to fix up after the fact
> if they really care.  Indeed, what *is* common practice in The Middle
> East, as regards how evenings get handled - as the tail of one day or
> the start of the next ?  Has secular society shifted the day boundary to
> midnight in practice ?
>

Indeed they do the simplification needed to adopt modern lifestyle. As far
as I know, modern lunar calendars being used in ME as official calendars,
have loads of simplifications applied compared to the original calendars.
The start of a month in Islamic calendar in Fiqh (regional calendar) is
based on observing new moon (by unaided eye) in the sky. That's why Saudi
Arabia sometimes celebrate end of Ramadan one day after Turkey, and one day
before Iran. That's why number of days differ from a year to another.
Though, the start of the day is 00:00 in modern calendars. And number of
days in months are fixed.

Anyway, the only country using lunar calendar (Saudi Arabia) just switched
to solar system (Gregorian). But we have to support Islamic calendar
(modern version) of course.

My current implementation looks like KCalendarSystem with some differences.
Calendar classes are subclassed from QAbstractCalendar, and have to
implement several methods. I tried to make this class as generic as
possible, there is no assumption on month days and number of months. I just
finished adding new methods to QDate and it's working for Jalali Calendar.

QDate d = QDate::currentDate();
int gyear1 = d.year();  // 2017
int gyear2 = d.year(QGregorianCalendar);  // 2017
int jyear1 = d.year(QJalaliCalendar()); // 1395
int jyear2 = d.year(QAbstractCalendar::fromName("jalali")); // 1395

I didn't touch Gregorian calculations in QDate, just added more functions.
Does removing static members count as an ABI change? I'm planning to move
all the math out of QDate into QGregorianCalendar.

About calendaring widgets, The widget obviously needs to keep information
on what calendar system it uses. I think it's possible to add members to
private API classes? After all it's **private** and there's only a pointer
to private API class in main class. So it would be A[BP]I compatible I
suppose. And also we need to add some functions to main class:

    void QDateEdit::setCalendar(QAbstractCalendar* cal); // maybe a QString?
    QAbstractCalendar* QDateEdit::calendar() const; // meh...

There is another issue: How do we add calendar localization to QLocale?

Cheers,
Soroush
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