qweb.xml

<openerp>
    <data>
       <template id="assets_backend" name="web_widget_datepicker_options assets" inherit_id="web.assets_backend">
            <xpath expr="." position="inside">
                <script type="text/javascript" src="/web_widget_datepicker_options/static/src/js/datepicker.js"></script>
            </xpath>
        </template>
    </data>
</openerp>