Field 'updatedAt' of type Datetime

[Anonymous]

When I am updating a lead via Rest API, Datetime field 'updatedAt' does not get updated.
Following is my partial Json object. I have many more fields and they all get updated except 'updatedAt'.
 "input":[{"updatedAt":"2015-02-06T14:36:07-08","firstName":"John","lastName":"Smith"}]

I noticed that this field is of type 'DateTime'. 
What Java datatype I should use in order to update this field? 
Appreciate any suggestions.
 

[Anonymous]

Hi Harshala

Some fileds are read only too.
I was confused before too.
You can check which filelds are read only at Lead Database "Lead Info" tab.
REST updatable fields are text box here.

[Anonymous]

This field is a meta data field. It updates based on other fields being updated.