Today I had to change Magento default products list sorting to sort by product’s date. In fact we don’t need to sort by date, just by entity_id which is product ID in Magento world. To accomplish this, we need to edit /app/code/core/Mage/Catalog/Block/Product/List.php

You have to find _beforeToHtml() function and change some things inside. First we need to define all available sorting options:

$orders = array('entity_id' => $this->__('Newest'), 'name' => $this->__('Name'), 'price' => $this->__('Price') );

then remove all getters of default sorting and order, and put those:

$toolbar->setAvailableOrders($orders);
$toolbar->setDefaultOrder('entity_id');
$toolbar->setDefaultDirection('desc');

so finally your _beforeToHtml() should looks like this one:

    protected function _beforeToHtml(){$toolbar = $this->getToolbarBlock();// called prepare sortable parameters$collection = $this->_getProductCollection();// use sortable parameters/*if ($orders = $this->getAvailableOrders()) {$toolbar->setAvailableOrders($orders);}if ($sort = $this->getSortBy()) {$toolbar->setDefaultOrder($sort);}if ($dir = $this->getDefaultDirection()) {$toolbar->setDefaultDirection($dir);}if ($modes = $this->getModes()) {$toolbar->setModes($modes);}*/$orders = array('entity_id' => $this->__('Newest'), 'name' => $this->__('Name'), 'price' => $this->__('Price') );$toolbar->setAvailableOrders($orders);$toolbar->setDefaultOrder('entity_id');$toolbar->setDefaultDirection('desc');// set collection to toolbar and apply sort$toolbar->setCollection($collection);$this->setChild('toolbar', $toolbar);Mage::dispatchEvent('catalog_block_product_list_collection', array('collection' => $this->_getProductCollection()));$this->_getProductCollection()->load();return parent::_beforeToHtml();}

from: http://www.blastar.biz/2011/07/29/how-to-sort-magento-products-by-date-added-as-default/