Database
[{"_id":12345,"docsArray":[{"createdDate":"2008-01-12T07:36:45-08:00"},{"createdDate":"2008-01-12T07:36:45-08:00"}]}]
Query
db.collection.update({_id:"12345"},[{$set:{docsArray:{$map:{input:"$docsArray",in:{createdDate:{$toDate:"$$this.createdDate"}}}}}}])