Ok sorry it's not good.
It makes further calculations wrong.
I correct it like this: in the function getWeekCalendar, I replace the line.
$setting->currentWeek = (int) $current->toFormat('%W');
by
$setting->currentWeek = (int) $current->toFormat('%W')+1;
But then I have to change the function getWeekSelect replacing this line:
$arr[] = JHTML::_('select.option', $week . ',' . $year, $week . '/' . $year);
by
$arr[] = JHTML::_('select.option', $week+1 . ',' . $year, $week+1 . '/' . $year);
But I'm not sure it will not affect the rest of the code. An answer from ARTIO would be great.