My form is not submitting what can i do there is 1 field named "done_at" is optional

This is my TaskController where done_at is optional and their is relation with 1 model named Staff.

public function create()
{
    $staffs = Staff::all();
    return view('tasks.create', compact('staffs'));
}
public function store(Request $request)
{
    $request->validate([
        'staff_id' => 'required',
        'task' => 'required',
        'done_at' => 'sometimes',
    ]);
    $task = Task::create([
        'staff_id' => $request->input('staff_id'),
        'task' => $request->input('task'),
        'done_at' => $request->input('done_at'),
    ]);

    return redirect()->route('tasks.index')->withSuccess('Done');
}

This is my Task model where their is relationship with staff model.

use HasFactory;

protected $table = 'tasks';

protected $fillable = [
    'staff_id',
    'task',
];

public function staff(){
    return $this->belongsTo(Staff::class);
}

This is my Staff model with has relationship with task model.

use HasFactory;

protected $table = 'staffs';

protected $fillable = [
    'name',
    'gender',
    'date_of_birth',
];

public function task() {
    return $this->hasMany(Task::class);
}

This is my tasks.create page where there is form which is not submitting the done_at field is optional.

<form method="post" action="">
        @csrf
        <table>
                <tr>
                    <td>Staff Name : </td>
                    <td>
                        <select name="staff_id" id="staff_id">
                            <option value="">Select One</option>
                            @foreach ($staffs as $staff)

                                <option value=""></option>

                            @endforeach
                        </select>
                    </td>
                </tr>
                <tr>
                    <td>Task : </td>
                    <td><input type="text" name="task" class="form-control"></td>
                </tr>
                <tr>
                    <td>Done At :</td>
                    <td><input type="time" name="done_at" class="form-control"></td>
                </tr>
            <td><button class="btn btn-primary" name="submit" type="submit" value="submit" id="submit">Submit</button></td>
        </table>
    </form>

This is my tasks table and this is structure of Task.

public function up()
{
    Schema::create('tasks', function (Blueprint $table) {
        $table->id();
        $table->bigInteger('staff_id')->nullable();
        $table->string('task')->nullable();
        $table->dateTime('done_at')->nullable();
        $table->timestamps();
    });
}

via Chebli Mohamed