##pnx-sprint

@berdir: I confirm the changes in the MR are still needed.

The issue I encountered was that on a form submit containing a datetime-local element, I always had an Exception "The date cannot be created from a format."

Dateime::valueCallback() was calling getHtml5DateFormat<code> which returns <code>DateFormat::load('html_datetime')->getPattern(); for datetime-local and datetime elements.
But 'html_datetime' (ie. Y-m-d\TH:i:sO) is not a valid format to pass to \DateTime::createFromFormat() in the following try / catch block.

Perhaps not handled in the most elegant way possible ; the fix I provided was very specific to the datetime-local element but datetime element might have the same problem.

Reroll of patch from #156 for D11.1

This has gone through multiple rounds of review, all threads have been resolved. I think it's ready to be RTBC.

Rebased MR 7340

The Needs Review Queue Bot → tested this issue. It no longer applies to Drupal core. Therefore, this issue status is now "Needs work".

This does not mean that the patch necessarily needs to be re-rolled or the MR rebased. Read the Issue Summary, the issue tags and the latest discussion here to determine what needs to be done.

Consult the Drupal Contributor Guide → to find step-by-step guides for working with issues.

nod_ → closed merge request !11971

Committed d381523 and pushed to 11.x. Thanks!