There seems to be no global setting allowing you to display dates in a numeric format by default at the moment, which is really unfortunate in my opinion.

You can still display individual dates numerically in XML using options :

<field name="your_date_field" options="{'numeric': true}"/>

If you want numeric dates and datetimes by default, you can load this code in a module loaded after web :

import { patch } from "@web/core/utils/patch";
import { DateTimeField } from "@web/views/fields/datetime/datetime_field";

patch(DateTimeField, {    
	​defaultProps: {        
	​	​showSeconds: false,
	​	​showTime: true,
	​	​// Numeric dates by default
	​	​numeric: true,    
	​},
});

(you can still switch to text format for individual XML fields by using 'numeric': false in options with this patch) 

Where can I get that javascript override to change the datepicker format? I prefer to have it in MM/dd/YYYY because in the report during the printing, old documents will not show the date.  (I can create a computed field though but takes extra coding).

​In the Philippines, the government requires that invoices will have full dates.  An invoice without full date is considered a violation of Internal Revenue policies.

We changed the date format in Odoo 19 based on user interface/user experience research showing that including a textual representation of the month is easier and more reliable to interpret than all-numeric formats. It reduces cognitive load on Users.

For any date field or date time field in Odoo 19 you can hover over it to see the traditional format:

Hi,

In Odoo 19, the datepicker defaults to MMM d (e.g., Sep 18) because it uses the Luxon library with localized formats, which can be confusing compared to the old MM/dd/yyyy. You can change this by editing the Language → Date Format in settings, but the picker may still show abbreviated months. The most reliable fix is a small JavaScript override in a custom module to force MM/dd/yyyy everywhere..

Hope it helps