首先在Article.php中已经将published_at设置成Carbon对象了，
public function setPublishedAtAttribute($date)
{
$this->attributes['published_at'] = Carbon::createFromFormat('Y-m-d', $date);
}
但是在ArticlesController中的index方法渲染view时，即显示主页面提示
Call to a member function diffForHumans() on a non-object
错误，index.blade.php中ul一块的代码是直接拷贝博客（手写了怕没找到错误）
@extends('app')
@foreach($articles as $article)
<article class="format-image group">
<h2 class="post-title pad">
<a href="/articles/{{ $article->id }}"> {{ $article->title }}</a>
</h2>
<ul class="post-meta pad group">
<li><i class="fa fa-clock-o"></i>{{ $article->published_at->diffForHumans() }}</li>
@if($article->tags)
@foreach($article->tags as $tag)
<li><i class="fa fa-tag"></i>{{ $tag->name }}</li>
@endforeach
@endif
</ul>
<div class="post-inner">
<div class="post-deco">
<div class="hex hex-small">
<div class="hex-inner"><i class="fa"></i></div>
<div class="corner-1"></div>
<div class="corner-2"></div>
</div>
</div>
<div class="post-content pad">
<div class="entry custome">
{{ $article->intro }}
</div>
<a class="more-link-custom" href="/articles/{{ $article->id }}"><span><i>更多</i></span></a>
</div>
</div>
</article>
@endforeach
求大大帮助！
