mb_grade로 member 테이블에서 하나 더늘렸는데

update g4_member mb_grade = '2012-10-10 19:30:44'

1064 : You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near '= '2012-10-10 19:30:44'' at line 2

error file : /adm/member_grade.php

<member_grade.php>

<?
include_once("./_common.php");

$sql = " update $g4[member_table]
mb_grade = '$g4[time_ymdhis]' ";
sql_query($sql);


?>

뭐가 문제인지 잘 모르겠습니다..