To fix this requires editing the libs/ubbthreads.inc.php. Line # may vary as I'm going off of the 7.1 code however it's around 1323. There is a block of code that looks like this:

Code
// Grab the time portion of their prefs
if( strpos( $timeformat, "H" ) !== false ) {
  $time_format = "H:i";
} else {
  $time_format = "h:i A";   
} // end if

That, needs to be changed to:

Code
// Grab the time portion of their prefs
if( strpos( $timeformat, "H" ) !== false ) {
  $time_format = "H:i";
} elseif ( strpos ( $timeformat, "G") !== false) {
  $time_format = "G:i";                  
} else {
  $time_format = "h:i A";   
} // end if