How to store Firebase server timestamp plus 30 seconds?

I'm storing objects on Firebase Realtime Database. An object looks like this:

import com.google.firebase.database.ServerValue;

public class Message {

    private Object timestamp = ServerValue.TIMESTAMP;
    private String source = "client";
    private String req;
    private String ack = null;
    private int pos;

    Message(){};    // needed for Firebase when converting DataSnapshot to Message object

    Message(String req, int pos) {
        this.req = req;
        this.pos = pos;
    }

    public Object getTimestamp() {
        return timestamp;
    }

    public String getSource() {
        return source;
    }

    public String getReq() {
        return req;
    }

    public String getAck() {
        return ack;
    }

    public int getPos() {
        return pos;
    }

}

I haven't tried it yet, though the timestamp should correctly be populated when storing the object onto Firebase.

Now my question is: how can I add an expiration attribute? The expiration would be equal to the TIMESTAMP plus 30 seconds like so:

private Object expiration = ServerValue.TIMESTAMP + <30 seconds>;