How to force calling of QWidget::paintEvent() when its hovered by other window?

I have a problem: I'm creating a widget which displays current date's day number. It's like a button, but it's not derived from QPushButton class. Just from QWidget. So I've reimplemented enterEvent(), leaveEvent(), mousePressEvent(), mouseReleaseEvent(). I do call update() inside these methods and widget has realistic button behavior (paintEvent() is reimplemented also). But when I change system date and hover that widget with other window, my widget doesn't get paintEvent() and the old date is displayed. Only when I place mouse over it, widget repaints it's contents. I guess there is a feature (like buffering) which paints old contents on hovering with other window to avoid unnecessary recalculations. But I need to disable it. Tried to set many attributes (the Qt::WidgetAttribute enum). But it doesn't work.

I think you should find a way to detect that the system time has changed and call update() when that happens. Any other method (like detecting the "hovering" of a window or waiting for a mouse event) will cause the update to occur too late.