TYPO3 Core

Here an example TCA column from tt_products:

        'endtime' => [
            'exclude' => 1,
            'label' => 'LLL:EXT:lang/Resources/Private/Language/locallang_general.xlf:LGL.endtime',
            'config' => [
                'type' => 'input',
                'size' => '8',
                'max' => '20',
                'eval' => 'date',
                'checkbox' => '0',
                'default' => '0',
                'range' => [
                    'upper' => mktime(0, 0, 0, 12, 31, 2020),
                    'lower' => mktime(0, 0, 0, date('m') - 1, date('d'), date('Y'))
                ]
            ]
        ],

This is correct according to the documentation: https://docs.typo3.org/typo3cms/TCAReference/ColumnsConfig/Type/Input.html#range

However, the core handles ranges (as integer) before it converts the date. Therefore, ranges are now broken for eval=date input columns.

This is caused by: https://docs.typo3.org/typo3cms/extensions/core/Changelog/8.5/Important-77702-CustomRenderTypesForDateAndDatetimeFieldsMustUseISO-8601.html?highlight=date
which also caused the distantly related bug https://forge.typo3.org/issues/82981

What happens is the following:

\TYPO3\CMS\Core\DataHandling\DataHandler::checkValueForInput() checks for the TCA range and compares the $value and $tcaFieldConf['range']['lower'] / ['upper'] both cast to "(int)".
As the incoming date is in ISO-8601, it will look like "2018-01-01T00:00:00+00:00". Casting it to integer to compare with the upper/lower range, the value becomes simply "2018". At this point, the lower end of the range ($tcaFieldConf['range']['lower']) is applied to $value.

Only afterwards, \TYPO3\CMS\Core\DataHandling\DataHandler::checkValue_input_Eval() is called, which then attempts to convert the ISO-8601 string to a unix timestamp - but at this point we already lost our original incoming ISO-8601 string, since the lower range value was already applied.