Datetime comparing when validating a Yii form

gistfile1.aw

<?php
public function rules()
{
    return array(   
        array(
            'end_time',
            'compare',
            'compareAttribute'=>'start_time',
            'operator'=>'>',
            'on'=>'insert',
            'message'=>'Ending time should be greater than starting time'
        ),
    );
} 

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