Creating topic with "Custom Creation Date"

Hello,

I'm importing topics into groups by a script. What I want to do is creating topics with specific dates. They already have a creation date on original system, so I want to import those with same dates. But when I create topics with variables shown below, it is created with "now" as creation date. How can I set creation date of topics? I also want to do it to comments posted to these topics.

 

-----  HERE, I'm creating topic but donno how to give creation time -----

$grouptopic = new ElggObject();
$grouptopic->subtype = "groupforumtopic";
$grouptopic->owner_guid = $owner_guid;
$grouptopic->container_guid = $group_guid;
$grouptopic->access_id = 2;
$grouptopic->title = "TEST Topic automatically created";

add_entity_relationship ($owner_guid, "member", $group_guid);

$grouptopic->save();

---- AND HERE, I'm adding comments to topic, but donno how to set creation time  ----

$grouptopic->status = "open";
$grouptopic->annotate('group_topic_post',"test mesaji","0", $owner_guid); 

Thanks a lot
Hakan