FYI, I applied the patch 3366615-15 on local site testing upgrade from D9 to D10 with 20+ forms and it works fine, no need to remove custom '#datepicker': true from each form. Much appreciated thank you.
Thanks @dromansab , that gets over the data entry error but the summary then shows the date as Mon, 08/28/2023 - 00:00 which is useless. It should be possible to set the format required, for me this is 28/08/2023. It worked fine in Drupal 9.
In reply to #3, #4 and #8, this is a real issue. I am converting a D9 site to D10, there are many forms with dates and this country uses the dd/mm/yyyy format. The data entry will only proceed if the date_date_format is Y-m-d, but then the date is wrong on the summary, the submission and subsequent processing.
Attached .yml file of a very simple test form and screenshots of the error and the Drupal date settings.
Happy to provide anything else that would help resolve this.