“1468777498”中使用的是什么时间格式？

Question

我有API检查已连接的手机到smsgateway.me。 这是我的API json响应

{"success":true,"result":{"id":"24678","name":"Device 24678","make":"Samsung","model":"GT-S7392","number":"",
"provider":"Telenor PK","country":"pk","connection_type":"2G","battery":"14","signal":"100","wifi":"1",
"lat":"0","lng":"0","last_seen":1468777498,"created_at":false}}

这是"last_seen":1468777498,"，但这次的类型是什么？

我怎么知道这是什么时间或者我可以解码/翻译它？另外，我想将last_seen提取到一个简单格式的PHP字符串中，如

$last_seen =  "2 minutes 30 seconds ago';