(not a) bug in QDate::weekNbr()?

[mzimmers]

Hi all -

I know it's hard to believe, but I ran across this while starting to implement a date/time picker. I wrote a routine to calculate the number of weeks (including partials) in a given month, and ran into this when testing March, 2020.

#include <QDateTime>

int weeksInMonth(int year, int month) {
    QDate qDate = QDate::currentDate();
    int day;
    int firstWeek;
    int lastWeek;
    int nbrWeeks = 0;
    int yearNbr;

    do {
        day = 1;
        qDate.setDate(year, month, day);
        firstWeek = qDate.weekNumber(&yearNbr);
        if (yearNbr != year) {
            firstWeek = 0;
        }
        qDebug().nospace() << "first week for " << year << "/" << month << " is " << firstWeek;

        day = qDate.daysInMonth();
        qDate.setDate(year, month, day);
        lastWeek = qDate.weekNumber(&yearNbr);
        if (yearNbr != year) {
            // this only can happen (to lastWeek) in December,
            // so we can derive the last week
            // based on the weekday of 12/1.
            if (qDate.dayOfWeek() >= 5) { // 5 here means Thursday
                lastWeek = firstWeek + 5;
            } else {
                lastWeek = firstWeek + 4;
            }
        }
        qDebug().nospace() << "last week for " << year << "/" << month << " is " << lastWeek;

        nbrWeeks = lastWeek - firstWeek + 1;
    } while (false);
    return nbrWeeks;
}

int main(int argc, char *argv[])
{
    int nbrWeeks;
    int year = 2020;
    int month = 3;
    nbrWeeks = weeksInMonth(year, month);
    qDebug().nospace() << "number of weeks in " << year << "/" << month << " is " << nbrWeeks;
    return 0;
}

Here's the output:

first week for 2020/3 is 9
last week for 2020/3 is 14
number of weeks in 2020/3 is 6

And here's the real thing:

Am I missing something, or is this a bug?

Thanks...

[Christian Ehrlicher]

Please read the docs:

"Returns the ISO 8601 week number (1 to 53)."

https://en.wikipedia.org/wiki/ISO_8601#Week_dates

"...beginning with Monday and ending with Sunday. "

Just because the US thinks they can ignore the iso everywhere doesn't mean the iso changes just because of them.

[mzimmers]

@Christian-Ehrlicher ah, yes. So, I guess I'll need to modify my routine to accept a start-of-week argument. Of course, this means I'll have to refine my calendar display as well. Thanks for pointing this out.