How to make controller to insert time on click and remove time on another click?

This is my controller where i want to know what is correct.

public function complete($id)
{
    $task = Task::find($id);
    if($task->done_at = '' or $task->done_at = null) {
        $task->done_at = date( 'Y-m-d H:i:s', strtotime( 'now' ) );

        $task->save();
    } elseif ($task->done_at !== null) {
        $task->done_at = null;

        $task->save();
    }

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

and this is my index page where there is form for clicking

<form action="" method="post">
                        @csrf
                        <button type="submit" name="submit"><p style="border-radius: 50%; color:red"></p></button>
                    </form>

via Chebli Mohamed