I ran into this exact same issue myself.  This was asked in 2013 in this post (https://piwigo.org/forum/viewtopic.php?id=21507).  There is no configuration option to change the date, but the function that generates it is "format_date" and it's very easy to modify.  At the time of this post, this function is in the file "piwigo/include/functions.inc.php".  Simply re-arrange the lines as you like.  Code begins around line 794 in the file.  Here's mine:

function format_date($original, $show=null, $format=null)
{
  global $lang;
  $date = str2DateTime($original, $format);
  if (!$date)
  {
    return l10n('N/A');
  }
  if ($show === null || $show === true)
  {
    $show = array('day_name', 'day', 'month', 'year');
  }
  // TODO use IntlDateFormatter for proper i18n
  $print = '';
  // Weekday
  if (in_array('day_name', $show))
    $print.= $lang['day'][ $date->format('w') ].' - ';
  // Month
  if (in_array('month', $show))
    $print.= $lang['month'][ $date->format('n') ].' ';
  // Day
  if (in_array('day', $show))
    $print.= $date->format('j').', ';
  // Year
  if (in_array('year', $show))
    $print.= $date->format('Y').' ';
  if (in_array('time', $show))
  {
    $temp = $date->format('H:i');
    if ($temp != '00:00')
    {
      $print.= $temp.' ';
    }
  }
  return trim($print);
}

If you have the option "Show the date period associated with each album..." under the menu "Configuration > Options > Display Tab", then you'll probably want to change the function just below it called "format_fromto" in the same functions.inc.php file.  Here's my hacked version:

function format_fromto($from, $to, $full=false)
{
  $from = str2DateTime($from);
  $to = str2DateTime($to);
  // Dates match.  Return only 1 of them.
  if ($from->format('Y-m-d') == $to->format('Y-m-d'))
  {
    return format_date($from);
  }
  // Dates do not match.  Return a range.
  else
  {
    $from_str = format_date($from);
    $to_str = format_date($to);
    $from_str = substr($from_str, strpos($from_str, " - ") + 3);
    $to_str = substr($to_str, strpos($to_str, " - ") + 3);
    return l10n('%s to %s', $from_str, $to_str);
  }
}

I don't know if it's possible, but a much nicer way to update these two functions would be to use a Personal plugin or such.  But...I don't know how to do that (replace one function with another through a plugin) so I'm stuck using this method.

Hope this helps.