How to save DateType in phpcassa?

Question

I am trying to save a DateType to cassandra database using phpcassa.

This is what I do:

$date = new DateTime();
$column_family_commentByPost = new ColumnFamily($pool, 'commentByPost');
$column_family_commentByPost->insert($my_uuid_string1, array($date->getTimestamp() => $my_uuid_string2));
$array = $column_family_commentByPost->get($my_uuid_string1);
var_dump($array);

I get the following error:

Notice: unserialize(): Error at offset 0 of 10 bytes in C:...\DateType.php on line 20

What is the correct way to save DateType using php?