I have noticed a faulty behavior in this situation: I have a node with a date field with optional end date. If I set the start date but not the end date the node is displayed in fullcalendar with the default length of 120 minutes, but if I resize It the end date is wrong It can even be less than the start date.
I solved this issue temporarily by changing fullcalendar module source starting from row 175 like this:

// Datestamp can't combine with words for strtotime, convert to ISO for now.
if ($format == DATE_FORMAT_UNIX) {
$old_start = date(DATE_FORMAT_ISO, $old_start);
$old_end = date(DATE_FORMAT_ISO, $old_end);
/*changes */
if($old_end <= $old_start)
$defaultEventMinutes = 120;
$old_end = date($format, strtotime($old_end . '+ ' . $defaultEventMinutes . ' minutes'));

/* end of changes */