请帮助解决这个错误
致命错误: 异常 'Exception',信息为 'DateTime::_construct() [datetime.--construct]: Failed to parse time string (--) at position 0 (-): Unexpected character',位于 Z:\home\plati\www\view.php 的第 110 行。堆栈跟踪: #0 Z:\home\plati\www\view.php(110): DateTime->_construct('--') #1 {main} thrown in Z:\home\plati\www\view.php on line 110
$newday = $a['dayz'];
$endmonth = $a['monthz'];
$newyear = $a['yearz'];
$date = new DateTime("$newyear-$endmonth-$newday");
$date->modify('+8 day');
$year = $date->format('Y');
$month = $date->format('m');
$day = $date->format('d');