TYPO3 Core

When a table has a configuration for a column to be readOnly and eval as datetime, the backend form generated a PHP Warning and shows no form:

$columns['activated_on'] = array(
    'exclude' => 1,
    'label' => 'LLL:EXT:sf_register/Resources/Private/Language/locallang_be.xml:fe_users.activated_on',
    'config' => array(
        'type' => 'input',
        'readOnly' => TRUE,
        'eval' => 'datetime',
    )
);

This happens because the FormEngine class tries to render the field and the value for a new field is the empty string:
TYPO3\CMS\Backend\Form\FormEngine:1503

This could be solved, if the itemFormElValue is cast to an integer, if a datetime format is detected.

You could also argue, that this is a misconfiguration in TCA, because adding 'default' => 0 also solves the problem:

$columns['activated_on'] = array(
    'exclude' => 1,
    'label' => 'LLL:EXT:sf_register/Resources/Private/Language/locallang_be.xml:fe_users.activated_on',
    'config' => array(
        'type' => 'input',
        'readOnly' => TRUE,
        'eval' => 'datetime',
        'default' => 0,
    )
);