Filter by timestamp in Object ID in MongoDB

Use something like this to filter based on timestamp embedded within MongoDB Object id: 

db.yourcoll.find({
  "$expr":{
    "$and":[
      {$gte:[{"$toDate":"$_id"}, "$c9_thismonth-2m"]},
      {someOtherField: 'someval'}
    ]
  }
},{timestamp:{$toDate:"$_id"}}})
)