ElasticSearch upsert document with complex script

[tour_young]

I create an index to store the duration of how long people watching a certain video, the index structure like this:

{
    "first_watch_time": "2019-04-01T02:17:14.004Z",
    "last_watch_time": "2019-04-01T02:18:24.004Z",
    "duration": 70
}

when I create document I use the user_id as the document _id. For example when Bob(user_id is 1) first watch the video, trigger an event, there is no document in the index, and so I create one for him:

PUT video_duration/video_duration/1
{
    "first_watch_time": event_time,
    "duration": 0
}

the field "first_watch_time" is when the event occurred. After a while when Bob click the pause button, trigger a new event, now I want to use the upsert syntax to update the duration:

POST video_duration/video_duration/1/_update
{
  "script": {
    "source": "ctx._source.duration += params.duration; ctx._source.last_watch_time = event_time",
    "lang": "painless",
    "params": {
      "duration": "duration(event_time - ctx._source['first_watch_time'])"
    }
  },
  "upsert": {
    "duration": 0,
    "first_watch_time": event_time
  }
}

now the question is I dont know how to write the params, and I find the value of the key in params seems to be a specific value number or string, and also I dont know how to subtract the two timestamp in script. Is this possible to use upsert to calculate the duration, or I have to first query the document, and the calculate the duration in memory and then use one more request to update it?