FWIW - I have been tasked to change the date format (to "day, mm/dd/yyyy" & "day, mmm dd, yyyy" and the time format (from 24-hour to 12-hour) throughout the forum. I was able to get the intended results by making changes to the following files:

[root]/administrator/components/com_fireboard/language/english.php - lines 49-51 (shown with my changes):
DEFINE('_FB_DT_DATE_FMT','%m/%d/%Y';
DEFINE('_FB_DT_TIME_FMT','%r';
DEFINE('_FB_DT_DATETIME_FMT','%m/%d/%Y %I:%M %p';
(NOTE: lines 52-89 appear to define the names of months and days-of-the-week.)

[root]/language/english.php - lines 422-429 (shown with my changes):
DEFINE('_DATE_FORMAT','l, F d, Y'; //Uses PHP's DATE Command Format - Depreciated
/**
* Modify this line to reflect how you want the date to appear in your site
*
*e.g. DEFINE("_DATE_FORMAT_LC","%A, %d %B %Y %H:%M"; //Uses PHP's strftime Command Format
*/
DEFINE('_DATE_FORMAT_LC',"%A, %B %d, %Y"; //Uses PHP's strftime Command Format
DEFINE('_DATE_FORMAT_LC2',"%A, %B %d, %Y %r";

[root]/components/com_fireboard/template/default/view.php - line 1007 (shown with my changes):
/*$msg_date = date(_DATETIME, $fmessage->time);*/
$msg_date = date('l, m/d/Y h:i A', $fmessage->time);
(NOTE: this was suggested previously in this thread)

[root]/components/com_fireboard/template/default/latestx.php - line 168 (shown with my changes):
echo "" . date('l, m/d/Y h:i A', $latestPostTime) . "";

This has worked for my Forum where dates/times are displayed. The last one (latestx.php) affects the display when clicking "Show Latest Posts".