How to calculate the 'Finish' time shown in Chrome DevTools Network tab from the HAR object?

I am creating a Chrome Extension. I retrieved the HAR object from the DevTools panel. I am able to get the 'Load' and 'DOMContentLoaded' time from the HAR object,

pages: [
  {
    "startedDateTime": "2019-03-07T22:16:02.333Z",
    "id": "page_2",
    "title": "https://xxxx",
    "pageTimings": {
      "onContentLoad": 1635.4740000006132,
      "onLoad": 2318.5020000000804
    }
  }
]

Now, how do I calculate the 'Finish' time from the HAR object which is 7.75 s as shown below?

So, I have figured it out,

FinishTime=(lastRequestStartedTime + lastRequestDuration) - (firstRequestStartedTime)

const requests=harObject.entries;

const lastRequest = requests.reduce(
    (a, b) => {
      return a.startedDateTime > b.startedDateTime ? a : b;
    }
  );

const firstRequest = requests.reduce(
    (a, b) => {
      return a.startedDateTime < b.startedDateTime ? a : b;
    }
  );

//Finish Time in Seconds
const finishTime=`((Date.parse(lastRequest.startedDateTime) + lastRequest.time
                  - Date.parse(firstRequest.startedDateTime))
                  / 1000).toFixed(2)