Thanks ! However I need to add a couple more to work:
$this->grocery_crud->set_js('assets/grocery_crud/js/jquery-1.7.1.min.js');
$this->grocery_crud->set_js('assets/grocery_crud/js/jquery_plugins/jquery-ui-1.8.10.custom.min.js');
$this->grocery_crud->set_js('assets/grocery_crud/js/jquery_plugins/jquery.ui.datetime.js');
$this->grocery_crud->set_js('assets/grocery_crud/js/jquery_plugins/config/jquery.datepicker.config.js');
$this->grocery_crud->set_js('assets/grocery_crud/js/jquery_plugins/config/jquery.datetime.config.js');
$this->grocery_crud->set_css('assets/grocery_crud/css/ui/simple/jquery-ui-1.8.10.custom.css');
$this->grocery_crud->set_css('assets/grocery_crud/css/jquery_plugins/jquery.ui.datetime.css');
That's what i meant, i was in hurry

but you can do with an even more simple line, try this :
$this->grocery_crud->change_field_type('date', 'datetime');
and your textbox(es) becoming a date picker, without adding more JS and CSS