Python Code :

class PurchaseOrder(models.Model):
_inherit = 'purchase.order'

date_approve_date = fields.Date(
string="Confirmation Date",
compute='_compute_date_approve_date',
store=True
)

@api.depends('date_approve')
def _compute_date_approve_date(self):
for record in self:
record.date_approve_date = record.date_approve.date() if record.date_approve else False

XML :
<record id="purchase_order_inherited" model="ir.ui.view">
<field name="name">purchase.order.inherited.custom.form</field>
<field name="model">purchase.order</field>
<field name="inherit_id" ref="purchase.purchase_order_form"/>
<field name="arch" type="xml">
</xpath>
<xpath expr="//div[@name='date_approve']/field[@name='date_approve']" position="attributes">
<attribute name="invisible">1</attribute>
</xpath>

<xpath expr="//group/label[@for='date_approve']" position="attributes">
<attribute name="for">date_approve_date</attribute>
</xpath>

<xpath expr="//div[@name='date_approve']/field[@name='date_approve']" position="before">
<field name="date_approve_date"/>
</xpath>
</field>
</record>