we added a custom column that has integer value. How can we now add a FROM-tO filter like below?

Is there a special command for this?

$this->addColumn('created_at', array(
    'header' => Mage::helper('sales')->__('Purchased On'),
    'index' => 'created_at',
    'type' => 'datetime',
    'width' => '100px',
    'filter_index' => 'main_table.created_at'
));