Get serverTime from firebase in grenwich time

Im using this method to get the time from the firebase server, but if a user have another TimeServer in their phone it cause troubles in my app. Anybody knows how can i do this works getting GreenWich meridian, or get in any country in the world the current time from the server in EUA, i know when user System.currentMilis() this take the Time from the cellphone. But how i can get the value from the double offset.

DatabaseReference offsetRef = FirebaseDatabase.getInstance().getReference(".info/serverTimeOffset");
offsetRef.addValueEventListener(new ValueEventListener() {

  public void onDataChange(DataSnapshot snapshot) {
    double offset = snapshot.getValue(Double.class);
    double estimatedServerTimeMs = System.currentTimeMillis() + offset;

  public void onCancelled(DatabaseError error) {
    System.err.println("Listener was cancelled");