phpBB: Custom user field values for type date are not converted

[Kirby]

Affected version

1.5.2

When importing from phpBB with a user profile field of type date, XenForo imports the existing value litereally:

if (isset($choiceLookUps[$fieldId]))
{
    $fieldInfo = $choiceLookUps[$fieldId];
    $fieldChoiceId = max(0, $user["pf_$fieldId"] - 1); // option ids are 0 keyed, values are 1 keyed

    if (isset($fieldInfo[$fieldChoiceId]))
    {
        $fieldValue = $fieldInfo[$fieldChoiceId];
    }
}
else
{
    // set the field value directly
    $fieldValue = $user["pf_$fieldId"];
}

This does not work properly as phpBB stores the value as j-n-Y while XenForo expects Y-m-d