Question: How can I get all the products within the orders of a determine time interval, lets say from January to March ?

Answer: 
Haven't tested the code, but it should work. And also, change the dates to whatever you like

```
$from = date('YYYY-MM-DD');
$to = date('YYYY-MM-DD');

Mage::getResourceModel('sales/order_item_collection')
->addFieldToFilter('created_at', array(
        'from' => $from,
        'to' => $to,
        'date' => true,
        ));

```
You can see this in admin-end under   

```
Reports -> Products -> Products Ordered

```