when the admin attempts to use the admin_config.php page to change the event start/end times, the datetime fields will be constrained by the daterange restriction in the settings page. if the system is set to not allow posts/items to be added with dates outside the event range, then the event start/end fields will have no further options available.
workaraound at the moment is to disable that restriction, change the event start/end, then re-enable the restriction again.