by Andreas Panagiotopoulos.
I have also experienced the same problem.
To overcome this issue you can change the step at time selector to one minute instead of five minutes. So you can set availability at 31 October, 23:59.
You can do this by editing MoodleQuickForm_date_time_selector function of lib/form/datetimeselector.php file, about line 83-84:
$this->_options = array('startyear' => $calendartype->get_min_year(), 'stopyear' => $calendartype->get_max_year(),
'defaulttime' => 0, 'timezone' => 99, 'step' => 5, 'optional' => false);
Change 'step' value to 1 instead of 5!
Does this work for you?
Regards,
Andreas