partner-contact/partner_contact_birthdate/views/res_partner.xml

<?xml version="1.0" encoding="utf-8"?>
<!-- License AGPL-3.0 or later (https://www.gnu.org/licenses/agpl.html).
     Copyright 2015 Grupo ESOC Ingeniería de Servicios, S.L.U. -->

<odoo>
    <record id="view_personal_information_birthdate_date" model="ir.ui.view">
        <field name="name">Birthdate Date field</field>
        <field name="model">res.partner</field>
        <field name="inherit_id" ref="partner_contact_personal_information_page.personal_information"/>
        <field name="arch" type="xml">
            <xpath expr="//group[@name='personal_information_group']">
                <field name='birthdate_date'/>
            </xpath>
        </field>
    </record>
</odoo>