Here is the portion of code which format the date:
$post_date = create_date_ip($config['default_dateformat'], $postrow[$i]['post_time'], $config['board_timezone']);
Other time formats can be found in language/lang_***/lang_main_settings.php:
$lang['ENCODING'] = 'UTF-8';
$lang['DIRECTION'] = 'ltr';
$lang['USER_LANG'] = 'en-gb';
$lang['HEADER_LANG'] = 'en-gb';
$lang['HEADER_LANG_XML'] = 'en-gb';
$lang['HEADER_LANG_JQUERY'] = 'en-GB';
$lang['LEFT'] = 'left';
$lang['RIGHT'] = 'right';
$lang['DATE_FORMAT'] = 'd F y H:i'; // This should be changed to the default date format for your language, php date() format
$lang['DATE_FORMAT_DATE'] = 'Y/m/d';
$lang['DATE_FORMAT_TIME'] = 'H.i';
$lang['DATE_FORMAT_MG'] = 'Y/m/d - H:i';
$lang['DATE_FORMAT_CHAT'] = 'Y/m/d - H.i.s';
$lang['DATE_FORMAT_VF'] = 'd M Y';
$lang['DATE_FORMAT_DATE_JQUI_JQ'] = 'yy/mm/dd'; // You can only use one of these three formats: dd/mm/yy, mm/dd/yy, yy/mm/dd
$lang['DATE_FORMAT_TIME_JQUI_JQ'] = 'hh:ss';
$lang['DATE_FORMAT_DATE_JQUI_PHP'] = 'Y/m/d'; // You can only use one of these three formats: d/m/Y, m/d/Y, Y/m/d
$lang['DATE_FORMAT_TIME_JQUI_PHP'] = 'H.i';
$lang['DATE_FORMAT_DATE_MYSQL_PHP'] = 'ymd'; // You can only use one of these three formats: dmy, mdy, ymd (this is used to convert dates from MySQL and PHP and vice versa, all dates are with leading zeroes)
$lang['DATE_FORMAT_BIRTHDAY'] = 'd F Y';
$lang['DATE_FORMAT_CALENDAR'] = 'd F Y'; // This should be changed to the default date format for your language, php date() format
$lang['JOINED_DATE_FORMAT'] = 'F Y'; // Date format of Joined date, php date() format
$lang['NUMBER_FORMAT_DECIMALS'] = 0;
$lang['NUMBER_FORMAT_DECIMALS_SEP'] = '.';
$lang['NUMBER_FORMAT_THOUSANDS_SEP'] = ',';
$lang['NUMBER_FORMAT_DATE_SEP'] = '/';
$lang['NUMBER_FORMAT_TIME_SEP'] = ':';
$lang['DIRECTION'] = 'ltr';
$lang['USER_LANG'] = 'en-gb';
$lang['HEADER_LANG'] = 'en-gb';
$lang['HEADER_LANG_XML'] = 'en-gb';
$lang['HEADER_LANG_JQUERY'] = 'en-GB';
$lang['LEFT'] = 'left';
$lang['RIGHT'] = 'right';
$lang['DATE_FORMAT'] = 'd F y H:i'; // This should be changed to the default date format for your language, php date() format
$lang['DATE_FORMAT_DATE'] = 'Y/m/d';
$lang['DATE_FORMAT_TIME'] = 'H.i';
$lang['DATE_FORMAT_MG'] = 'Y/m/d - H:i';
$lang['DATE_FORMAT_CHAT'] = 'Y/m/d - H.i.s';
$lang['DATE_FORMAT_VF'] = 'd M Y';
$lang['DATE_FORMAT_DATE_JQUI_JQ'] = 'yy/mm/dd'; // You can only use one of these three formats: dd/mm/yy, mm/dd/yy, yy/mm/dd
$lang['DATE_FORMAT_TIME_JQUI_JQ'] = 'hh:ss';
$lang['DATE_FORMAT_DATE_JQUI_PHP'] = 'Y/m/d'; // You can only use one of these three formats: d/m/Y, m/d/Y, Y/m/d
$lang['DATE_FORMAT_TIME_JQUI_PHP'] = 'H.i';
$lang['DATE_FORMAT_DATE_MYSQL_PHP'] = 'ymd'; // You can only use one of these three formats: dmy, mdy, ymd (this is used to convert dates from MySQL and PHP and vice versa, all dates are with leading zeroes)
$lang['DATE_FORMAT_BIRTHDAY'] = 'd F Y';
$lang['DATE_FORMAT_CALENDAR'] = 'd F Y'; // This should be changed to the default date format for your language, php date() format
$lang['JOINED_DATE_FORMAT'] = 'F Y'; // Date format of Joined date, php date() format
$lang['NUMBER_FORMAT_DECIMALS'] = 0;
$lang['NUMBER_FORMAT_DECIMALS_SEP'] = '.';
$lang['NUMBER_FORMAT_THOUSANDS_SEP'] = ',';
$lang['NUMBER_FORMAT_DATE_SEP'] = '/';
$lang['NUMBER_FORMAT_TIME_SEP'] = ':';