Hi @sebastian ,
If you want to change only dashboard filter date format then you need to override bagisto vue datetime.vue component,
please follow these link
https://github.com/bagisto/bagisto/issues/1992
after that only need to change the format ref - https://flatpickr.js.org/examples/
And if you want to change whole admin section date format which is your requirement Y-m-d to d.m.Y, then simply update some code of datetime.vue which is inside packages/Webkul/Ui/src/Resources/assets/js/components/datetime.vue
then simply run npm run watch for compiling the vue assets inside the Ui package root.
Thanks