Eloquent & Carbon Time Difference

I have a basic table which captures time and attendance.

I'm trying to out put the total hours on site, from my eloquent query:

$attendance = DB::table('staff_attendances')
        ->whereBetween('in_date', array($date1, $date2))->where('staff_id', $ID)->select('first_name', 'last_name', 'in_date', 'out_date', 'in_time', 'out_time')->get();

I get the following back in json.

[{"first_name":"TestFirst","last_name":"TestLast","in_date":"2016-01-30","out_date":"2016-01-30","in_time":"11:40:34","out_time":"12:41:10"},

{"first_name":"TestFirst","last_name":"TestLast","in_date":"2016-01-30","out_date":"2016-01-30","in_time":"13:02:27","out_time":"14:02:32"}]

Which method would be best to out put total hours on site?

I've tried using carbon with the following:

$startTime = Carbon::parse($attendance->in_time);
$finishTime = Carbon::parse($attendance->out_time);
$totalDuration = $finishTime->diffInHours($startTime);

But I get "Trying to get property of non-object"

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