[Oblivion Knight]

It's because you have the yesterday/today timestamp (or at least it looks as though you have with the code you have provided).

Try replacing this:

function vbdate($format,$timestamp) { 
  global $bbuserinfo,$timeoffset, $dateformat, $timeformat; 

  if ($dateformat==$format) { 
    $todaystamp=mktime(); 
    $todaydate=date($format,$todaystamp+($bbuserinfo['timezoneoffset']-$timeoffset)*3600); 
    $yestdate=date($format,(($todaystamp-86400)+($bbuserinfo['timezoneoffset']-$timeoffset)*3600)); 
    $date = date($format,$timestamp+($bbuserinfo['timezoneoffset']-$timeoffset)*3600);

With this:

function vbdate($format,$timestamp) { 
  global $bbuserinfo,$timeoffset, $minuteshift, $dateformat, $timeformat; 

  if ($dateformat==$format) { 
    $todaystamp=mktime(); 
    $todaydate=date($format,($todaystamp+($minuteshift * 60))+($bbuserinfo['timezoneoffset']-$timeoffset)*3600); 
    $yestdate=date($format,((($todaystamp-86400)+($minuteshift * 60))+($bbuserinfo['timezoneoffset']-$timeoffset)*3600)); 
    $date = date($format,($timestamp+($minuteshift * 60))+($bbuserinfo['timezoneoffset']-$timeoffset)*3600);

It's untested, but I think it should work..