src/Type/DateType.php
public function encode($value, Field $field, Codec $codec = null)
    {
        if ($value instanceof \DateTimeInterface) {
            return $value->format('Y-m-d');
        }

        return null;
    }
public function encode($value, Field $field, Codec $codec = null)
    {
        if ($value instanceof \DateTimeInterface) {
            return $value->format('Y-m-d');
        }

        return null;
    }
{@inheritdoc}