TYPO3 Core

Test it:

form definition:

type: Form
identifier: test
label: test
prototypeName: standard
renderables:
  -
    type: Page
    identifier: page-1
    label: Step
    renderables:
      -
        type: DatePicker
        identifier: datepicker-1
        label: 'Date picker'
        properties:
          dateFormat: Y-m-d
          enableDatePicker: false
          displayTimeSelector: false

Type x within the input and submit the form.
The error message is 'The date "%s" was not recognized (for format "%s").' but should be 'The date "x" was not recognized (for format "Y-m-d").'

Patch set 2 for branch master of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/56328

Patch set 3 for branch master of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/56328