How to convert Jalali date to Gregorian date

[Saviz]

I have a project where I need to interact with a MySQL database and manipulate dates. The problem, however, is that, as far as I can tell, MySQL only accepts dates in the Gregorian calendar. However, my users want to view and input dates in the Jalali calendar. To address this, I decided to make the dates viewable and editable in the Jalali date format but store and retrieve them in the Gregorian date format in the back-end.

Assuming I have access to the year, month, and day, I can convert to Jalali like this:

void gregorianToJalali(int year, int month, int day)
{
    QCalendar calendar(QCalendar::System::Jalali);

    QDate gregorianDate(year, month, day);

    QCalendar::YearMonthDay ymd = calendar.partsFromDate(gregorianDate);

    qDebug() << QString("%1/%2/%3")
                    .arg(ymd.year, 4, 10, QChar('0'))
                    .arg(ymd.month, 2, 10, QChar('0'))
                    .arg(ymd.day, 2, 10, QChar('0'));
}

However, the problem is that I do not know how to convert from Jalali to Gregorian:

void jalaliToGregorian(int year, int month, int day)
{
    QCalendar calendar(QCalendar::System::Gregorian);

    QDate jalaliDate(year, month, day); // This does not work, because QDate uses Gregorian dates by default.

    QCalendar::YearMonthDay ymd = calendar.partsFromDate(jalaliDate);

    qDebug() << QString("%1/%2/%3")
                    .arg(ymd.year, 4, 10, QChar('0'))
                    .arg(ymd.month, 2, 10, QChar('0'))
                    .arg(ymd.day, 2, 10, QChar('0'));
}

Obviously, this does not work. I have two questions regarding this:

1- How do I convert between the two calendar types?
2- Is it better to have a VARCHAR field in MySQL to store Jalali dates, since MySQL does not natively support them?

[hskoglund]

Try replacing that first line
QCalendar calendar(QCalendar::System::Gregorian);
with
QCalendar calendar(QCalendar::System::Jalali);

[Christian Ehrlicher]

You should take a look at https://doc.qt.io/qt-6/qcalendar.html to convert from your calendar system to gregorian calendar.

[Saviz]

@Christian-Ehrlicher I read the documentation multiple times before posting my question, but I find it difficult to understand. I would greatly appreciate it if you could point out what I might be doing wrong in my code. That’s why I took the time to ask for help.

[Christian Ehrlicher]

I would go with https://doc.qt.io/qt-6/qcalendar.html#dateFromParts-1

[Saviz]

@Christian-Ehrlicher said in How to convert Jalali date to Gregorian date:

I would go with https://doc.qt.io/qt-6/qcalendar.html#dateFromParts-1

That is exactly what I thought I should use as well. However, I’ve encountered a problem:

void jalaliToGregorian(int year, int month, int day)
{
    QCalendar calendar(QCalendar::System::Gregorian);

    QDate gregorianDate = calendar.dateFromParts(year, month, day);

    qDebug() << QString("%1/%2/%3")
                    .arg(gregorianDate.year(), 4, 10, QChar('0'))
                    .arg(gregorianDate.month(), 2, 10, QChar('0'))
                    .arg(gregorianDate.day(), 2, 10, QChar('0'));
}

According to the code, I pass in a Jalali date 1403/10/15. I expect to obtain the corresponding Gregorian date, which should be similar to January 4, 2025. Instead, I get this:

"1403/10/15"

This result is obviously incorrect, but I can’t figure out why.

[hskoglund]

Try replacing that first line
QCalendar calendar(QCalendar::System::Gregorian);
with
QCalendar calendar(QCalendar::System::Jalali);

[Saviz]

@hskoglund this did indeed work. and the results are correct:

"2025/01/04"

Thank you for your help.

Here is the code if anyone is curious:

void jalaliToGregorian(int year, int month, int day)
{
    QCalendar calendar(QCalendar::System::Jalali);

    QDate gregorianDate = calendar.dateFromParts(year, month, day);

    qDebug() << QString("%1/%2/%3")
                    .arg(gregorianDate.year(), 4, 10, QChar('0'))
                    .arg(gregorianDate.month(), 2, 10, QChar('0'))
                    .arg(gregorianDate.day(), 2, 10, QChar('0'));
}