使用AttributeBehavior给model属性设置 值 [ 2.0 版本 ]

AttributeBehavior
这个看不懂怎么用呀，

[
    'class' => AttributeBehavior::className(),
    'attributes' => [
        ActiveRecord::EVENT_BEFORE_INSERT => ['validity_start','validity_end'],
        ActiveRecord::EVENT_BEFORE_UPDATE => ['validity_start','validity_end'],
    ],
    'value' => function ($event) {
        return time();
    },
],

我想通过AttributeBehavior 来自动设置validity_start和validity_end，页面中是字符串的日期2016-04-12这样的，我想通过这个behavior，来转换为time时间戳后设置值。