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).

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