IOS的FCM通知生存时间

Question

I am trying to set time-to-live for IOS notification using FCM from server.

{
    "data":{
    "message":"Experience the blissful Kailash Mansarovar the serenity of its beautiful places..Book Holiday now. T&C Apply.",
    "createdOn":"1537273860715",
    "notificationId":"35",
    "title":"Divine visit to Mansarovar",
    "notificationtype":"package",
    "imageUrl":"",
    "redirect":"PKG000543"
    },
    "registration_ids":[
    "fhsTDf1G0Kk:APA91bEzzuR7yE6HPCii0pHEbcWXTslcj6MImtlQ23I65OU6dJLBOyH79Gy6APhRSXW-7mQDQ81NBMvDPTsfkrlHXjM4bryuIRSCV_ljWIyA1mAF5eJf5O35gV4p8m7daDP4WRoPi8Sz",
    "fhsTDf1G0Kk:APA91bEzzuR7yE6HPCii0pHEbcWXTslcj6MImtlQ23I65OU6dJLBOyH79Gy6APhRSXW-7mQDQ81NBMvDPTsfkrlHXjM4bryuIRSCV_ljWIyA1mAF5eJf5O35gV4p8m7daDP4WRoPi8So",
    "fhsTDf1G0Kk:APA91bEzzuR7yE6HPCii0pHEbcWXTslcj6MImtlQ23I65OU6dJLBOyH79Gy6APhRSXW-7mQDQ81NBMvDPTsfkrlHXjM4bryuIRSCV_ljWIyA1mAF5eJf5O35gV4p8m7daDP4WRoPi8Sz"
    ],
     "apns":{
      "headers":{
        "apns-expiration":"1604750400"
      }
    }
    }

无法理解apns到期时间格式。

请让我知道1604750400是几秒钟还是几毫秒或任何其他格式？