Laravel 5 ORM update request, like date_add - php

Question

I beg to excuse me for my poor english.
So, I have Laravel 5 ORM, and i need to make request that should add date to some rows, like MySQL DATE_ADD. Input is single date interval and array of id's, output is rows of database, that was changed by adding date interval. Ideally, it should be one ORM request. I know that it is possible to use "bad" way and get all rows, update it in a code, and insert to database, but imho it's not good.
I hope answer will be link to some help site or some code if it's does not complicate you. Thanks for your attention!
public function update($id)
{
$user_id = Auth::user()->id;
$rep_task = RepTask::find($id);
$cl_task = \Request::has('json') ? json_decode(\Request::input('json'),true) : \Request::all();
$ids = [];
$task_id = $rep_task->task_id;
$rep_tasks = RepTask::where('task_id', '=', $task_id)
->select('id')
->get();
$new_date = date_create_from_format(self::DATE_FULLCALENDAR_FORMAT, $cl_task['new_date']);
$selected_task_date = date_create_from_format(self::DATE_MYSQL_FORMAT, $rep_task->start_date);
$diff = date_diff($selected_task_date, $new_date);
if(\Request::has('json'))
{
$ids = [1,2,3]; //this for easy understanding
DB::table('rep_task')
->whereIn('id', $ids)
->update(['start_date' => DB::raw('DATE_ADD(start_date, INTERVAL ' . $diff->d . ' DAY)')]);
$out_json = ['updated' => $ids];
return json_encode($out_json, JSON_UNESCAPED_UNICODE);
}
else
{
$start_date = 0;
$end_date = 0;
if (!isset($cl_task['name']) || !isset($cl_task['text']))
return "{'error':'columns are not defined'}";
if (isset($cl_task['start_date']) && isset($cl_task['end_date']))
{
$dt = date_create_from_format(self::DATE_FULLCALENDAR_FORMAT, $cl_task['start_date']);
$start_date = $dt->format(self::DATE_MYSQL_FORMAT);
$dt = date_create_from_format(self::DATE_FULLCALENDAR_FORMAT,$cl_task['end_date']);
$end_date = $dt->format(self::DATE_MYSQL_FORMAT);
}
$rep_task->name = $cl_task['name'];
$rep_task->text = $cl_task['text'];
$rep_task->start_date = $start_date;
$rep_task->end_date = $end_date;
$rep_task->users_id = $user_id;
$rep_task->save();
}
$user_id = Auth::user()->id;
$tasks = Task::getAllTasksByUserFullcalendar($user_id);
return view(
'task.index',
[
'tasks' => $tasks
]
);
}