Dynamically how to change the app invite code?

Currently what I am doing is,When the user registers on the app a unique number is getting generated and this code is saved in Firebase and in invitation text I am using this code to invite other users to join my circle .

Date myDate = new Date();
        SimpleDateFormat format1 = new SimpleDateFormat("yyyy-MM-dd  hh:mm:ss a", Locale.getDefault());
        String date = format1.format(myDate);
        Random r = new Random();
        int n = 100000 + r.nextInt(900000);
        String code = String.valueOf(n);

on clicking register button this 'code' is getting saved to firebase.

now i want to make an expiry period for this code.That is I need to create a new random number every 5dyas. How to accomplish this.This is my first app.Please help me.

below I am showing the invitation activity code

else if (id == R.id.inviteMembers) {

            reference.addValueEventListener(new ValueEventListener() {
                @Override
                public void onDataChange(DataSnapshot dataSnapshot) {
                  String  code=dataSnapshot.child(uid).child("code").getValue(String.class);
                    Intent myIntent = new Intent(Intent.ACTION_SEND);
                    myIntent.setType("text/plain");
                    myIntent.putExtra(Intent.EXTRA_TEXT,"Hi join my Circle to share location.My Circle Code is"+code);
                    startActivity(myIntent.createChooser(myIntent,"Share Using: "));
                    locreference.child("online").setValue("true");
                }