#DevDaysLisbon

@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.