Filter Magento collection by date

mage_collection_filter_date.php

<?php
$collection->addFieldToFilter('start_date',
    array(
        array('to' => Mage::getModel('core/date')->gmtDate('Y-m-d')),
        array('start_date', 'null' => '')
    )
);
$collection->addFieldToFilter('end_date',
    array(
        array('gteq' => Mage::getModel('core/date')->gmtDate('Y-m-d')),
        array('end_date', 'null' => '')
    )
);