Time is not update in larvel 5

I have a form in which i input two fields Activity_datetime_from and Activity_datetime_to. the problem is that when i update the form if i update these two fields then update function is working fine but when i update other fields not these two than it update these two fields automatically to 0000-00-00 00:00:00 like that. Here is the code of my update function:

public function update(UpdateActivity $request, $id)
{
    $activity = Activity::find($id);

    if(!$activity) {
        return response()->json(['message'=>'Data not found', 404]);
    }
    $activity->update($request->except(['activity_type', 'activity_picture']));

    if($request->hasFile('activity_picture')) {
        $activity_picture = $request->file('activity_picture');
        $activity_picture_name = time().'.'.$activity_picture->getClientOriginalExtension();
        $path = public_path('Storage/ActivityImages');
        $activity_picture->move($path, $activity_picture_name);
        $activity->activity_picture = 'Storage/ActivityImages/'.$activity_picture_name;
    }

    if ($request->has('activity_address')) {
        $location = $request->input('activity_address');
        $client = new \GuzzleHttp\Client();

        $geocoder = new Geocoder($client);

        $geocoder->setApiKey(config('geocoder.key'));

        $address = $geocoder->getCoordinatesForAddress($location);

        $latitude = $address['lat'];
        $longitude = $address['lng'];
    }
    $activity->latitude = $latitude;
    $activity->longitude = $longitude;
    $activity->save();

    return redirect()->route('activities.index')
                    ->with('success','Activity updated successfully');
}

Here is my view edit.blade.php

 <div class="form-group">
        {!!Form::label('activity_datetime_from','Activity From*:',['class'=>'col-md-2'])!!}
        <div class="col-md-10">
          <p></p>
          {!!Form::date('activity_datetime_from', null, ['class'=>'form-control','activity_datetime_from'])!!}
          <!-- {!!Form::text('dateTime-local', null, ['class'=>'form-control',])!!} -->
        </div>
      </div>

      <div class="form-group">
        {!!Form::label('activity_datetime_to','Activity To*:',['class'=>'col-md-2'])!!}
        <div class="col-md-10">
          <p></p>
          {!!Form::date('activity_datetime_to', null, ['class'=>'form-control','activity_datetime_to'])!!}
        </div>
      </div>

      <div class="form-group">
        {!!Form::label('activity_address','Address*:',['class'=>'col-md-2'])!!}
        <div class="col-md-10">
          {!!Form::text('activity_address', null, ['class'=>'form-control','required'])!!}
        </div>
      </div>

I want if i will update other fields than these two field will not change it contains the existing value. Please help Thanks in Advance.

via Chebli Mohamed