eZ\Publish\Core\Search\Common\FieldValueMapper\DateMapper::map PHP Method

    public function map(Field $field)
    {
        if (is_numeric($field->value)) {
            $date = new DateTime("@{$field->value}");
        } else {
            try {
                $date = new DateTime($field->value);
            } catch (Exception $e) {
                throw new InvalidArgumentException('Invalid date provided: ' . $field->value);
            }
        }
        return $date->format('Y-m-d\\TH:i:s\\Z');
    }