Add default datetime format for workbook

src/xlsx/xlsxworkbook.cpp

@@ -51,6 +51,7 @@ WorkbookPrivate::WorkbookPrivate(Workbook *q) :
 
     strings_to_numbers_enabled = false;
     date1904 = false;
+    defaultDateFormat = QStringLiteral("dd/mm/yyyy hh:mm");
     activesheet = 0;
     firstsheet = 0;
     table_count = 0;
@@ -105,6 +106,19 @@ bool Workbook::isStringsToNumbersEnabled() const
     return d->strings_to_numbers_enabled;
 }
 
+QString Workbook::defaultDateFormat() const
+{
+    Q_D(const Workbook);
+    return d->defaultDateFormat;
+
+}
+
+void Workbook::setDefaultDateFormat(const QString &format)
+{
+    Q_D(Workbook);
+    d->defaultDateFormat = format;
+}
+
 void Workbook::defineName(const QString &name, const QString &formula)
 {
 

src/xlsx/xlsxworkbook.h

@@ -63,6 +63,8 @@ public:
     void setDate1904(bool date1904);
     bool isStringsToNumbersEnabled() const;
     void setStringsToNumbersEnabled(bool enable=true);
+    QString defaultDateFormat() const;
+    void setDefaultDateFormat(const QString &format);
 
 private:
     friend class Package;

src/xlsx/xlsxworkbook_p.h

@@ -48,6 +48,7 @@ public:
 
     bool strings_to_numbers_enabled;
     bool date1904;
+    QString defaultDateFormat;
 
     int x_window;
     int y_window;

src/xlsx/xlsxworksheet.cpp

@@ -421,6 +421,10 @@ int Worksheet::writeDateTime(int row, int column, const QDateTime &dt, Format *f
     if (d->checkDimensions(row, column))
         return -1;
 
+    if (!format) {
+        format = d->workbook->createFormat();
+        format->setNumberFormat(d->workbook->defaultDateFormat());
+    }
     d->cellTable[row][column] = QSharedPointer<XlsxCellData>(new XlsxCellData(dt, XlsxCellData::DateTime, format));
     d->workbook->styles()->addFormat(format);