str = QString::fromUtf8((const char *)str.sprintf("%d. deň v roku", i));

invalid cast from type 'QString' to type 'const char*'

str = QString::fromUtf8(str.sprintf("%d. deň v roku", i));

no matching function for call to 'QString::fromUtf8(QString&)'

QString QString::fromUtf8 ( const char * str, int size = -1 )