Are you looking for the definition of those functions? - if yes they are in this file
Code:
\components\com_jem\classes\output.class.php
In function formatDateTime() there is else clause around line 1195 that unconditionally adds text to output if your date is NULL
Code:
$output .= '<span class="jem_date-1">'.JText::_('COM_JEM_OPEN_DATE').'</span>';
If you simply want to override "Open Date" text, the text constant COM_JEM_OPEN_DATE can be found in relevant language folder\file in this folder
Code:
\components\com_jem\language\