Test

gistfile1.js

FeedItem.aggregate({
  // you want to group all the things by status key
  $group: {
    _id: '$statusName', // assuming you have `statusName` field in your collections, prefix with `$`.
    timeOn: {
        // $cond takes 3 arguments as an array, cond expression, true expression and false expression
        $cond: [
                // if event was active, we want to add the milliseconds to timeOn
                { $eq: [ "$wasActive", true ] }, 
                
                // so let's compute actual difference
                $sum: {
                  $millisecond: {
                    $substract: ['$startDate', '$endDate']
                  }
                }, 
                
                // otherwise, 
                $substract: {
                  $millisecond: {
                    $substract: ['$startDate', '$endDate']
                  }
                }
        ]
    }
  }
})