请问V2EX API提供的json数据中created元和last_modified元代表的一串数字是什么含义？

http://www.v2ex.com/api/replies/show.json?topic_id=2
返回的json数据如下：
[{

"id" : 2,
"title" : "访问速度？",
"url" : "http://www.v2ex.com/t/2",
"content" : "大家目前访问这个网站的速度如何？\u000D\u000A\u000D\u000A我还没有把任何 cache 打开。",
"content_rendered" : "大家目前访问这个网站的速度如何？\u000D\u000A\u003Cbr /\u003E\u000D\u000A\u003Cbr /\u003E我还没有把任何 cache 打开。",
"replies" : 43,
"member" : {
"id" : 1,
"username" : "Livid",
"tagline" : "Beautifully Advance",
"avatar_mini" : "http://cdn.v2ex.com/avatar/c4ca/4238/1_mini.png?m=1353664649",
"avatar_normal" : "http://cdn.v2ex.com/avatar/c4ca/4238/1_normal.png?m=1353664649",
"avatar_large" : "http://cdn.v2ex.com/avatar/c4ca/4238/1_large.png?m=1353664649"
},
"node" : {
"id" : 1,
"name" : "babel",
"title" : "Project Babel",
"url" : "http://www.v2ex.com/go/babel",
"topics" : 1040,
"avatar_mini" : "http://www.v2ex.com/static/img/node_mini.png",
"avatar_normal" : "http://www.v2ex.com/static/img/node_normal.png",
"avatar_large" : "http://www.v2ex.com/static/img/node_large.png"
},
"created" : 1272207731,
"last_modified" : 1335106658,
"last_touched" : 1294932324

}]

但是一直搞不清"created" : 1272207731,
"last_modified" : 1335106658,
"last_touched" : 1294932324
的含义，为什么是这么一串数字？