Yii关系中两个日期之间的差异

How can I perform a DATEDIFF function in CDbCriteria relations in yii

$criteria->with = array(
    'company'=>array(
        'condition'=>"user_type=2 AND approval_status='A'", 
        'select'=>'user_company'
    ), 
    'subscription'=>array(
        'condition'=>"subscription_status='A'", 
        'select'=>'DATEDIFF(DATE(NOW()),DATE(subscription_startdate))'
    )
);

I tried using the above code, Then got an error saying DATEDIFF not found in particular table. So I add subscription relation as follows

'return array(
        'company'=>array(self::BELONGS_TO,'Users',array('user_id'=>'user_id')),
        'subscription'=>array(self::STAT,'SubscriptionDetails','subscription_startdate'),           
        );

Now also showing the below error

CDbCommand failed to execute the SQL statement: SQLSTATE[42S22]: Column not found: 
1054 Unknown column 'Array' in 'field list'

Is this the right way to perform date difference in yii relations? Thanks in Advance