joomla3.1 - Strict Standards: convertDate() -

strict standards: non-static method joomleaguehelper::convertdate() should not called statically, assuming $this incompatible context in

<td class="center"> <?php echo jhtml::calendar(   joomleaguehelper::convertdate($date), 'match_date'.$row->id, 'match_date'.$row->id, '%d-%m-%y',   'size="9"  tabindex="2" ondblclick="copyvalue(\'match_date\')"      onchange="document.getelementbyid(\'cb'.$i.'\').checked=true"');  ?>  </td> 

if calling convertdate statically, define static method:

public static function convertdate() {}

if not writer of code, try setting php error reporting level other strict standards.