Firebase Storage - How to attach timestamp to the pushed record?

Below is the code snippet for it

public class Member {
    private String name; //name of the member
    private String ID; //email for e.g
    private Map timestamp;
    private String fireBaseKey;

    public String getName(){return this.name;}
    public String getID(){return this.ID;}
    public Map getTimestamp() {return timestamp;}
    public String getFireBaseKey(){return this.fireBaseKey;}

    public void setName(String name){this.name = name;}
    public void setID(String ID){this.ID = ID;}
    public void setTimestamp(Map ts) {this.timestamp= ts;}
    public void setFireBaseKey(String key){this.fireBaseKey = key;}
}

Map map = new HashMap();

map.put("timestamp", ServerValue.TIMESTAMP);

Member member = new  Member();

member.setID(userID);

member.setName(name);

member.setTimestamp(ServerValue.TIMESTAMP);

DatabaseReference databaseReference = FirebaseDatabase.getInstance().getReference();

        String pathName = APP_NAME + "/" + conversation.getConversationPath();

        final DatabaseReference messagesRef = databaseReference.child(pathName).child(conversation.getFbRecordKey()).child("members");

        messagesRef.push().setValue(member);