JS Bin// source https://jsbin.com/wavofodoza

jsbin.wavofodoza.js

//console.log(stringDateToEpoch("2016-01-31T21:52:02.607Z"))

var obj = { "rank_history": [
      {"date": "2016-02-17T04:27:16.773Z","rank": 121},
      {"date": "2016-02-15T05:37:16.773Z","rank": 122},
      [1455763092,153],
      [1455761092,154],
      {"date": "2016-02-14T02:27:16.773Z","rank": 125},
      [1455781509,156]
    ]
}

var rankHistory = obj.rank_history // array
var new_format = []

new_format = _.map(rankHistory, function(value,key){
  if (value.date && value.rank){
    return ([stringDateToEpoch(value.date),value.rank])
  } else {
    return ([value[0],value[1]])
  }
  
  //console.log(key,value)
})
//rankHistory = new_format
console.log(new_format)

function stringDateToEpoch(string){
  return Math.round((new Date(string)).getTime() / 1000)

}