Worksheet::writeDate() is very slow. Add a separate dateToNumber() function to improve speed.

[archibalduk]

The following line in Worksheet::writeDate() is causing a bottleneck when writing fairly large numbers of QDates to a worksheet:

double value = datetimeToNumber(QDateTime(dt, QTime(0,0,0)), d->workbook->isDate1904());

The solution is to exclude QDateTime from the calculation when dealing with QDates. So the above line in Worksheet::writeDate() would be replaced with this:

double value = dateToNumber(dt);

The new dateToNumber() function in xlsxutility.cpp would be as follows:

qint64 dateToNumber(const QDate &dt) const
{
    const QDate epoch(1899, 12, 31);
    const QDate excel_date(epoch.daysTo(dt));

    // Account for Excel erroneously treating 1900 as a leap year
    if (excel_date > 59) // 59 = 28-Feb-1900
        return excel_date + 1;
    else
        return excel_date;
}

I'm not 100% clear on the Workbook::isDate1904() function, but the above approach works in my own extension of QXlsx. Outputting ~250,000 cells containing QDates now takes about 12 seconds instead of around 5-10 minutes.