laravel groupBy date with where condition date not working

I have some data like bellow and i want to show them as daily summation by duration column(in sec unit) from 7 days ago to 1 day ago (until yesterday), i use groupBy date and when i used where condition nothing returned, what's my wrong?

sorry for weak eng.lang., thank you

start_time                    end_time                duration
2018-05-13 13:06:18           2018-05-13 13:15:07     529
2018-05-13 13:15:35           2018-05-13 13:29:05     810
...
2018-05-14 10:16:36           2018-05-14 10:16:54     18
...

and my controller function code is

public function userWeeklyTimes()
  {
    $times = Auth::user()->times()
                          ->whereBetween('start_time', [Carbon::now()->subDays(7), Carbon::yesterday()])
                          ->groupBy(DB::raw('DATE(start_time)'))
                          ->selectRaw(
                              'sum(duration) as duration'
                          )->pluck('duration');

    return $times;
  }

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