datetime-local element is failing on Datetime::valueCallback()

Well then it seems that there's a bug somewhere

In Datetime::processDatetime() doc says:

  /**
   * [...]
   * #date_date_format: A date format string that describes the format that
   *     should be displayed to the end user for the date. When using HTML5
   *     elements the format MUST use the appropriate HTML5 format for that
   *     element, no other format will work. See the
   *     DateFormatterInterface::format() function for a list of the possible
   *     formats and HTML5 standards for the HTML5 requirements. Defaults to the
   *     right HTML5 format for the chosen element if an HTML5 element is used,
   *     otherwise defaults to DateFormat::load('html_date')->getPattern().
   * [...]
   */

And in DateFormatterInterface::format() :

  /**
   * [...]
   * @param string $type
   *   (optional) The format to use, one of:
   *   - One of the built-in formats: 'short', 'medium',
   *     'long', 'html_datetime', 'html_date', 'html_time',
   *     'html_yearless_date', 'html_week', 'html_month', 'html_year'.
   *   - The name of a date type defined by a date format config entity.
   *   - The machine name of an administrator-defined date format.
   *   - 'custom', to use $format.
   *   Defaults to 'medium'.
   * @param string $format
   *   (optional) If $type is 'custom', a PHP date format string suitable for
   *   input to date(). Use a backslash to escape ordinary text, so it does not
   *   get interpreted as date format characters.
   * [...]
   */

No matter what the value I give to that "#date_date_format" ("long", "short", "medium", 'html_datetime', "d/m/Y H:i", "Y-m-d H:i", 'd/m/Y', Y-m-d, etc.) - and I've got the same result if I don't set that "#date_date_format" (which leads to a Y-m-d default value) :

1. the input is always asking for a "d/m/Y H:i" as you can see on the attached screenshot
2. when I choose a date and time and submit the form, I am always reaching Datetime::valueCallback() with the following values:
   • $input =

      array:1 [▼
       "date" => "2025-02-09T12:00"
     ]
     

   • $date_input = "2025-02-09T12:00"
   • $time_input = ""
   • $date_format = "Y-m-d\TH:i:sO"
   • $time_format = ""
3. Then I am always catching the Exception with the message "The date cannot be created from a format."

Sorry quietone #8 I didn't realised I changed the version

Ok now I undestand why I've got this behavior no matter what I pass to '#date_date_format'
It's because of the Datetime::valueCallback() variables attributions made just before the try/catch:
$date_format = $element['#date_date_format'] != 'none' ? static::getHtml5DateFormat($element) : '';

protected static function getHtml5DateFormat($element) {
  switch ($element['#date_date_element']) {
    case 'date':
      return DateFormat::load('html_date')->getPattern();

    case 'datetime':
    case 'datetime-local':
      return DateFormat::load('html_datetime')->getPattern();

    default:
      return $element['#date_date_format'];
  }
}

and as cilefen said in #4, 'html_datetime' (ie. "Y-m-d\TH:i:sO") is not a valid format to pass to \DateTime::createFromFormat()

MR provides a fix for the Datetime::ValueCallback() method.

Few thoughts:
- We should maybe find a way to fix it in a less-dirty way (ie. not with that ugly "if" condition)
- It only fixes the problem for the datetime-local element but datetime element also uses the 'html_datetime' format and would break the same way
- Datetime::processDatetime() needs to be updated as well, $element['#value'] is now not correct to display the submitted value in the input done
- We should introduce tests

#13 Yes I wanted some feedback, thank you.

I updated the issue summary ; hopefully it is clear enough and will help resolving the issue.

Added kernel tests for datetime-local

Thanks smustgrave.
I applied your suggestions except for the first one since it's how it's done on other core KernelTests.
IMHO it will be easier to maintain the "form + tests" mix rather than to declare the form in another location even if it sounds prettier.