web_tree_date_search/purchase.xml

<?xml version="1.0" encoding="utf-8"?>
<openerp>
    <data noupdate="1">

        <record id="purchase.purchase_form_action" model="ir.actions.act_window">
            <field name="res_model">purchase.order</field>
            <field name="context">{'dates_filter' : ['date_order']}</field>
        </record>

    </data>
</openerp>