use the function

_inherit="sale.order"

today_date = fields.Datetime('Date', default=datetime.today())

@api.multi
    @api.onchange('date_order')
    def _check_date(self):
        for order in self:
            start_date = order.date_order
            end_date = order.today_date

            if start_date < end_date:
                raise Warning(_('Please select proper Order date.'))
                order.update({'date_order': datetime.today()})