Assigning _timestamp to a property

Hi,

I am trying to assign _timestamp to a property when I post to ElasticSearch. Something is wrong and I have been staring at this for a while now. ThnX in advance for any pointers.

GET /persons/_mapping/family

"family": {
"_timestamp": {
"enabled": true,
"store": true,
"path": "family.create_date",
"format": "yyyy-MM-dd HH:mm:ss",
"ignore_missing": true
},
"properties": {
"about": {
"type": "string"
},
"age": {
"type": "integer"
},
"create_date": {
"type": "date",
"format": "dateOptionalTime"
},
"first_name": {
"type": "string"
},
"interests": {
"type": "string"
},
"last_name": {
"type": "string"
}
}
}

POST /persons/family
{
"first_name" : "me_myself",
"last_name" : "_and_I",
"age": "00",
"about" : "I love to figure things out",
"interests": [ "photography", "running", "chocolate", "music" ]
}

GET /persons/family/_search

  "hits": [
     {
        "_index": "persons",
        "_type": "family",
        "_id": "AU3HyMph3okB74YfFVvn",
        "_score": 1,
        "_source": {
           "first_name": "me_myself",
           "last_name": "_and_I",
           "age": "00",
           "about": "I love to figure things out",
           "interests": [
              "photography",
              "running",
              "chocolate",
              "music"
           ]
        }
     },